437. Path Sum III (Easy)

https://leetcode.com/problems/path-sum-iii/

You are given a binary tree in which each node contains an integer value.

Find the number of paths that sum to a given value.

The path does not need to start or end at the root or a leaf, but it must go downwards (traveling only from parent nodes to child nodes).

The tree has no more than 1,000 nodes and the values are in the range -1,000,000 to 1,000,000.

Example:

root = [10,5,-3,3,2,null,11,3,-2,null,1], sum = 8

      10
     /  \
    5   -3
   / \    \
  3   2   11
 / \   \
3  -2   1

Return 3. The paths that sum to 8 are:

1.  5 -> 3
2.  5 -> 2 -> 1
3. -3 -> 11

Solutions

class Solution {
    public int pathSum(TreeNode root, int sum) {

        // keep the track of visiting path
        LinkedList<Integer> path = new LinkedList<>();

        return traverse(root, path, sum);
    }

    private int traverse(TreeNode root, LinkedList<Integer> path, int target) {
        int ans = 0;
        if (root == null) {
            return ans;
        }

        path.add(root.val);

        int sum = 0;
        int len = path.size();
        for (int i = len - 1; i >= 0; i--) {
            sum += path.get(i);
            if (sum == target) {
                ans++;

                // do not break once found, there may contains 0 values that brings in another combination
            }
        }

        ans += traverse(root.left, path, target);
        ans += traverse(root.right, path, target);

        path.removeLast();

        return ans;
    }
}

Incorrect Solutions

References

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