LeetCode link

first thought

  • DFS

solution

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
public class Solution {
    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        List<List<Integer>> results = new ArrayList<>();
        if (root != null) {
            List<Integer> path = new ArrayList<>();
            path.add(root.val);
            this.helper(root, sum - root.val, path, results);
        }
        return results;
    }
    private void helper(TreeNode root, int sum, List<Integer> path, List<List<Integer>> results) {
        if (sum == 0) {
            if (root.left == null && root.right == null) { // 题目要求必须是leaf
                List<Integer> temp = new ArrayList<Integer>(path);
                results.add(temp);
            }
            return;
        }
        if (root.left != null) {
            path.add(root.left.val);
            this.helper(root.left, sum - root.left.val, path, results);
            path.remove(path.size() - 1);
        }
        if (root.right != null) {
            path.add(root.right.val);
            this.helper(root.right, sum - root.right.val, path, results);
            path.remove(path.size() - 1);
        }
    }
}

problem

wrong answer

reason

not all the inputs are positive, so the sum may be 0 during some steps, we must traverse all the paths.

modification

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
public class Solution {
    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        List<List<Integer>> results = new ArrayList<>();
        if (root != null) {
            List<Integer> path = new ArrayList<>();
            path.add(root.val);
            this.helper(root, sum - root.val, path, results);
        }
        return results;
    }
    private void helper(TreeNode root, int sum, List<Integer> path, List<List<Integer>> results) {
        if (sum == 0 && root.left == null && root.right == null) {
            List<Integer> temp = new ArrayList<Integer>(path);
            results.add(temp);
            return;
        }
        if (root.left != null) {
            path.add(root.left.val);
            this.helper(root.left, sum - root.left.val, path, results);
            path.remove(path.size() - 1);
        }
        if (root.right != null) {
            path.add(root.right.val);
            this.helper(root.right, sum - root.right.val, path, results);
            path.remove(path.size() - 1);
        }
    }
}