[LeetCode] Path Sum

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.

 

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AC code如下，但是我不理解为什么 root==NULL && sum==0 时要返回false？？

考虑中间节点，只有一个左子树，右子树为NULL，所以只能到左子树继续判断了。。所以为NULL时要返回false吧，我想

 

class Solution {
    public:
        bool hasPathSum(TreeNode *root, int sum) {
            if(root == NULL)
            {   
                return false;
            }   

            if(root->left == NULL && root->right == NULL) //判断叶节点
            {
                if(root->val == sum)
                    return true;
                return false;
            }

            if ( hasPathSum(root->left, sum - root->val) // 递归
             || hasPathSum(root->right, sum - root->val) )
                return true;
            return false;
        }   
};