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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