Leetcode之Path Sum

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.

Note: A leaf is a node with no children.

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.

代码：

/**
 * 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:
    bool hasPathSum(TreeNode* root, int sum) {
        if(!root)return false;
        if(!root->left&&!root->right){
            if(sum==root->val)return true;
            else return false;
        }
        bool b1=false,b2=false;
        int value=sum-root->val;
        if(root->left){
            b1=hasPathSum(root->left,value);
        }
        if(root->right){
            b2=hasPathSum(root->right,value);
        }
        
        return b1||b2;
    }
};

想法：

考虑极端情况