Path sum

Q: 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.

A： 深度搜索。注意节点的val值有正有负，原先以为节点值都为正数，做了个剪枝：root->val>sum.

    bool hasPathSum(TreeNode *root, int sum) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(root==NULL)
            return false;
        
        if(root->val==sum&&!root->left&&!root->right)
            return true;
        else
            return hasPathSum(root->left,sum-root->val)||hasPathSum(root->right,sum-root->val);
        
    }