437. 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

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/path-sum-iii
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class Solution {
    public int pathSum(TreeNode root, int sum) {
        if (root == null) {
            return 0;
        }
        int ans = 0;
        ans += count(root, 0, sum);
        ans += pathSum(root.left, sum);
        ans += pathSum(root.right, sum);
        return ans;
    }

    public int count(TreeNode root, int tmp, int target) {
        int ans = 0;
        if (tmp +  root.val == target) {
            ans++;
        }
        if (root.left != null) {
            ans += count(root.left, tmp + root.val, target);
        }
        if (root.right != null) {
            ans += count(root.right, tmp + root.val, target);
        }
        return ans;
    }
}