leetcode 437. Path Sum III(路径和)(DFS)

问题描述：

  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.

思路：

  首先很明显是采用深度有限搜索DFS。针对每一个结点，可以把它当作根结点，只要求出从根结点出发的路径数和其左右结点出发的路径数即可。那如何去求某一个结点出发的路径数？利用深度有限搜索进行检查。

代码：

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */class Solution {    public int pathSum(TreeNode root, int sum) {        if(root == null)             return 0;        return dfs(root, sum) + pathSum(root.left, sum) + pathSum(root.right, sum);    }    private int dfs(TreeNode root, int sum){        if(root == null) return 0;        int num = 0;        if(sum == root.val)            num++;        num += dfs(root.left, sum - root.val);        num += dfs(root.right, sum - root.val);        return num;    }}