112.Path Sum

problems:
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.
solution:

class Solution {
public:
    bool hasPathSum(TreeNode* root, int sum) {
        if(!root) return false;//空节点的情况
        if(!root->left && !root->right) //只有根节点的情况
        {
            if(root->val != sum)
                return false;
            else
                return true;
        }
        bool a=hasPathSum(root->left,sum-root->val);
        bool b=hasPathSum(root->right,sum-root->val);
        
        if(a || b) return true;
        else return false;
    }
};