leetcode：Path Sum （路径之和） 【面试算法题】

题目：

Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.

For example:

Given the below binary tree and 
sum = 22
,

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \      \
        7    2      1

return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.

题意判断从根节点到叶子节点的路径和是否有等于sum的值。

叶子节点就是没有子节点的节点，判断叶子节点的当前值是否等于当前的sum。

若不是叶子节点，将当前sum减去当前节点的值，递归求解。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool hasPathSum(TreeNode *root, int sum) {
        if(!root)return false;
        if(sum==root->val&&root->left==NULL&&root->right==NULL)return true;
        return (root->left && hasPathSum(root->left,sum-root->val) )
             ||(root->right && hasPathSum(root->right,sum-root->val));
    }
};
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