代码随想录第二十九天

Posted on In

复习

77 组合

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22

class Solution {
    List<List<Integer>> result = new ArrayList<>();
    List<Integer> path = new ArrayList<>();
    public List<List<Integer>> combine(int n, int k) {
        backTracking(n, k ,1);
        return result;
    }

    private void backTracking(int n, int k, int start){
        if(path.size() == k){
            result.add(new ArrayList<>(path));
            return;
        }

        for(int i = start; i <= n - (k - path.size()) + 1; i++){
            path.add(i);
            backTracking(n , k, i + 1);
            path.remove(path.size() - 1);
        }
    }
}

17 电话号码的字母组合

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
class Solution {
    String[] numString = new String[] {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};

    List<String> result = new ArrayList<>();
    StringBuilder sb = new StringBuilder();
    public List<String> letterCombinations(String digits) {
        if (digits == null || digits.length() == 0) return result;
        backTracking(digits, 0);
        return result;
    }

    private void backTracking(String digits, int start){
        if (sb.length() == digits.length()){
            result.add(sb.toString());
            return;
        }
        String str = numString[digits.charAt(start) - '0'];
        for (int i = 0; i< str.length(); i++){
            sb.append(str.charAt(i));
            backTracking(digits, start + 1);
            sb.deleteCharAt(sb.length() - 1);
        }
    }
}

216 组合总和III

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
class Solution {
    List<List<Integer>> result = new ArrayList<>();

    List<Integer> path = new ArrayList<>();
    public List<List<Integer>> combinationSum3(int k, int n) {
        backTracking(k , n, 1);
        return result;
    }

    private void backTracking(int k, int n , int start){
        if (n < 0) return;
        if (n == 0 && path.size() == k){
            result.add(new ArrayList<>(path));
            return;
        }
        for (int i = start; i <= 9 - (k - path.size()) + 1; i++){
            path.add(i);
            n -= i;
            backTracking(k, n, i + 1);
            n += i;
            path.remove(path.size() - 1);
        }
    }
}

39 组合总和

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
class Solution {
    List<List<Integer>> result = new ArrayList<>();
    List<Integer> path = new ArrayList<>();
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        if (candidates == null || candidates.length == 0) return result;
        Arrays.sort(candidates);
        backTracking(candidates, target, 0);
        return result;
    }

    private void backTracking(int[] candidates, int target, int start){
        if (target < 0) return;
        if (target == 0){
            result.add(new ArrayList<>(path));
            return;
        }

        for (int i = start; i < candidates.length; i++){
            path.add(candidates[i]);
            target -= candidates[i];
            backTracking(candidates, target, i);
            target += candidates[i];
            path.remove(path.size() - 1);
        }
    }
}

40 组合总和II

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
class Solution {
    List<Integer> path = new ArrayList<>();
    List<List<Integer>> result = new ArrayList<>();

    boolean[] used;
    public List<List<Integer>> combinationSum2(int[] candidates, int target) {
        used = new boolean[candidates.length];
        Arrays.sort(candidates);
        Arrays.fill(used, false);
        backTracking(candidates, target, 0);
        return result;
    }

    private void backTracking(int[] candidates, int target, int start){
        if (target == 0){
            result.add(new ArrayList<>(path));
            return;
        }

        for (int i = start; i < candidates.length; i++){
            if (target < 0) break;
            if (i > 0 && candidates[i] == candidates[i - 1] && !used[i - 1]) continue;
            path.add(candidates[i]);
            used[i] = true;
            target -= candidates[i];
            backTracking(candidates, target, i + 1);
            target += candidates[i];
            used[i] = false;
            path.remove(path.size() - 1);
        }
    }
}

131 分割回文串

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
class Solution {
    List<List<String>> result = new ArrayList<>();
    List<String> path = new ArrayList<>();
    public List<List<String>> partition(String s) {
        if (s == null || s.length() == 0) return result;

        backTracking(s, 0);
        return result;
    }

    private void backTracking(String s, int start){
        if (start == s.length()){
            result.add(new ArrayList<>(path));
            return;
        }

        for (int i = start; i < s.length(); i++){
            if (isPalindrome(s, start, i)){
                path.add(s.substring(start, i + 1));
                backTracking(s, i + 1);
                path.remove(path.size() - 1);
            }
        }
    }

    private boolean isPalindrome(String s, int start, int end){
        while (start <= end){
            if (s.charAt(start) != s.charAt(end)) return false;
            start++;
            end--;
        }
        return true;
    }
}

93 复原ip地址

注意：
判断数字问题
判断最后一段符合逻辑
注意对s的操作

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
class Solution {
    List<String> result = new ArrayList<>();
    public List<String> restoreIpAddresses(String s) {
        if (s.length() == 0 || s.length() > 12) return result;
        backTracking(s, 0, 0);
        return result;
    }

    private void backTracking(String s, int start, int points){
        if (points == 3){
            if (isValid(s, start, s.length() - 1)){
                result.add(s);
            }

            return;
        }

        for (int i = start; i < s.length(); i++){
            if (isValid(s, start, i)){
                s = s.substring(0, i + 1) + "." + s.substring(i + 1);
                points++;
                backTracking(s, i + 2, points);
                points--;
                s = s.substring(0, i + 1) + s.substring(i + 2);
            } else break;
        }
    }

    private boolean isValid(String s, int start, int end){
        if (start > end) return false;
        if (s.charAt(start) == '0' && start != end) return false;
        int num = 0;
        for (int i = start; i <= end; i++){
            if (s.charAt(i) - '0' > 9 || s.charAt(i) - '0' < 0) return false;
            num = num * 10 + s.charAt(i) - '0';
            if (num > 255) return false;
        }
        return true;
    }
}

78 子集

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
class Solution {
    List<List<Integer>> result = new ArrayList<>();
    List<Integer> path = new ArrayList<>();
    public List<List<Integer>> subsets(int[] nums) {
        backTracking(nums, 0);
        return result;
    }

    private void backTracking(int[] nums, int start){
        result.add(new ArrayList(path));
        for (int i = start; i < nums.length; i++){
            path.add(nums[i]);
            backTracking(nums, i + 1);
            path.remove(path.size() - 1);
        }
    }
}

90 子集II

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
class Solution {
    List<List<Integer>> result = new ArrayList<>();

    List<Integer> path = new ArrayList<>();

    boolean[] used;
    public List<List<Integer>> subsetsWithDup(int[] nums) {
        used = new boolean[nums.length];
        Arrays.fill(used, false);
        Arrays.sort(nums);
        backTracking(nums, 0);
        return result;
    }

    private void backTracking(int[] nums, int start){
        result.add(new ArrayList(path));
        for (int i = start; i < nums.length; i++){
            if (i > 0 && nums[i] == nums[i - 1] && !used[i - 1]) continue;
            path.add(nums[i]);
            used[i] = true;
            backTracking(nums, i + 1);
            used[i] = false;
            path.remove(path.size() - 1);
        }
    }
}

491 递增子序列

利用在回溯函数里面的hashset或者int[] 记录本层通过的点
利用if语句判断是否递增，或者是否是重复的值

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
class Solution {
    List<List<Integer>> result = new ArrayList<>();
    List<Integer> path = new ArrayList<>();
    
    public List<List<Integer>> findSubsequences(int[] nums) {
        backTracking(nums, 0);
        return result;
    }

    private void backTracking(int[] nums, int start){
        if (path.size() >= 2){
            result.add(new ArrayList<>(path));
        }
        int[] used = new int[201];
        for (int i = start; i < nums.length; i++){
            if (!path.isEmpty() && path.get(path.size() - 1) > nums[i] || used[nums[i] + 100] == 1  ) continue;
            path.add(nums[i]);
            used[nums[i] + 100] = 1;
            backTracking(nums, i + 1);
            path.remove(path.size() - 1);
        }
    }

    private boolean isNonDecreasing(List<Integer> nums){
        for (int i = 1; i < nums.size(); i++){
            if (nums.get(i) < nums.get(i - 1)){
                return false;
            }
        }

        return true;
    }
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23

class Solution {
    List<List<Integer>> result = new ArrayList<>();
    List<Integer> path = new ArrayList<>();
    public List<List<Integer>> findSubsequences(int[] nums) {
        backTracking(nums, 0);
        return result;
    }
    private void backTracking(int[] nums, int start){
        if (path.size() >= 2){
            result.add(new ArrayList<>(path));
        }
        Set<Integer> set = new HashSet<>();
        for (int i = start; i < nums.length; i++){
            if (!path.isEmpty() && path.get(path.size() - 1) > nums[i] || set.contains(nums[i])) continue;
            set.add(nums[i]);
            path.add(nums[i]);
            backTracking(nums, i + 1);
            path.remove(path.size() - 1);
        }
    } 
}

46 全排列

第一种 利用used记录path里面有没有nums[i]

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27

class Solution {
    List<List<Integer>> result = new ArrayList<>();
    List<Integer> path = new ArrayList<>();
    boolean[] used;
    public List<List<Integer>> permute(int[] nums) {
        used = new boolean[nums.length];
        backTracking(nums);
        return result;
    }

    private void backTracking(int[] nums){
        if (path.size() == nums.length){
            result.add(new ArrayList<>(path));
            return;
        }

        for (int i = 0; i < nums.length; i++){
            if (used[i]) continue;
            used[i] = true;
            path.add(nums[i]);
            backTracking(nums);
            used[i] = false;
            path.removeLast();
        }
    }
}

第二种直接通过检查path里面有没有nums[i] 元素判断
List 有个 removeLast 方法

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
class Solution {
    List<List<Integer>> result = new ArrayList<>();
    List<Integer> path = new ArrayList<>();
    public List<List<Integer>> permute(int[] nums) {
        backTracking(nums);
        return result;
    }

    private void backTracking(int[] nums){
        if (path.size() == nums.length){
            result.add(new ArrayList<>(path));
            return;
        }

        for (int i = 0; i < nums.length; i++){
            if (path.contains(nums[i])) continue;
            path.add(nums[i]);
            backTracking(nums);
            path.removeLast();
        }
    }
}

47 全排列II

去重代码中
!used[i - 1] 是树层去重
used[i - 1] 是树枝去重
要确定used[i] 是否为false， 因为i一直是从0开始的

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
class Solution {
    List<List<Integer>> result = new ArrayList<>();
    List<Integer> path = new ArrayList<>();

    boolean[] used;
    public List<List<Integer>> permuteUnique(int[] nums) {
        used = new boolean[nums.length];
        Arrays.sort(nums);
        backTracking(nums);
        return result;
    }

    private void backTracking(int[] nums){
        if (path.size() == nums.length){
            result.add(new ArrayList<>(path));
            return;
        }
        for (int i = 0; i < nums.length; i++){
            if (i > 0 && nums[i] == nums[i - 1] && !used[i - 1]) continue;

            if (used[i] == false){
                used[i] = true;
                path.add(nums[i]);
                backTracking(nums);
                path.removeLast();
                used[i] = false;
            }
        }
    } 
}