[LeetCode] PathSum

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.

思路：DFS.时间复杂度O(n), 空间复杂度O(lgN)

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class Solution {
public:
    bool hasPathSum(TreeNode *root, int sum) {
        if (root == NULL) {
            return false;
        }
        
        if (root->left == NULL && root->right == NULL) {
            return root->val == sum;
        }
        
        return hasPathSum(root->left, sum - (root->val)) ||
                hasPathSum(root->right, sum - (root->val));
    }
};