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

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MarkDone复制粘贴原题比较麻烦，换成TinyMCE的编辑器了。

这个题先用list试了一下，没发现什么好办法。直接看答案，发现是直接做。。。好吧。

public class Solution {
    int res = 0;
    public int pathSum(TreeNode root, int sum) {
        if (root == null) return 0;
        pathSum(root.left, sum);
        pathSum(root.right, sum);
        helper(root, sum);
        return res;
        
    }

    
    public void helper(TreeNode root, int sum) {
        if (root == null) return;
        if (sum - root.val == 0) res++;
        helper(root.left, sum-root.val);
        helper(root.right, sum-root.val);
        
        
    }
}

　　

寒一个，二轮电面应该是挂了，心好痛啊。。。。