437. Path Sum III

Problem

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

Solution

使用两个递归

• 外层递归用于遍历树中的每一个节点
• 内层递归用于求从当前节点开始的路径数

/**
 * 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)
            return 0;

        return paths(root,sum) + pathSum(root->left,sum) + pathSum(root->right,sum);
    }
    int paths(TreeNode* root,int sum)
    {
        if(!root)
            return 0;
        int ret = 0;

        //因为并不要求以叶子节点结束
        if(root->val == sum)
        {
            ++ret;    
        }

        ret += paths(root->left,sum - root->val);
        ret += paths(root->right,sum - root->val);

        return ret;
    }
};