leetcode 112.路径总和

　　简单地dfs一遍即可。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
int sum,num;
bool flag;

void dfs(TreeNode* now){
    if (now==NULL) return ;
    num+=(now->val);
    if (num==sum&&(now->left)==NULL&&(now->right)==NULL){
        flag=true;
        return ;
    }
    dfs(now->left);
    if (flag)   return ;
    dfs(now->right);
    num-=(now->val);
}

class Solution {
public:
    bool hasPathSum(TreeNode* root, int targetSum) {
        sum=targetSum,num=0,flag=false;
        dfs(root);
        return flag;
    }
};