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
解题思路：暴力搜索，寻找以每个节点为根结点，到达叶子节点之前看会不会出现sum的值。

/**
 * 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 sumUp(TreeNode* root,int pre,int &sum){
        if(!root)return 0;
        int current=pre+root->val;
        return (sum==current)+sumUp(root->left,current,sum)+sumUp(root->right,current,sum);
    }
    int pathSum(TreeNode* root, int sum) {
       if(!root)return 0;
       return sumUp(root,0,sum)+pathSum(root->left,sum)+pathSum(root->right,sum);
    }
};