[leetcode]437. Path Sum III

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本文解析了 LeetCode 上的 437. 路径总和 III 问题，介绍了如何寻找二叉树中所有节点路径数值之和等于指定值的路径数量。通过递归算法实现，无需路径一定从根节点到叶节点。

[leetcode]437. Path Sum III

Analysis

everyday is the first day of your rest life—— [间歇性迷茫+1~]

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.
给定一个二叉树和一个数字，看二叉树中有多少条路径上的数字之和等于该数字，这里的路径不一定非要从根节点到叶子节点。

Implement

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
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);
    }
    int DFS(TreeNode* root, int sum){
        int res = 0;
        if(root == NULL)
            return res;
        if(root->val == sum)
            res++;
        res += DFS(root->left, sum-root->val);
        res += DFS(root->right, sum-root->val);
        return res;
    }
};