[leetcode] 112 Path Sum

/*
http://leetcode.com/onlinejudge#question_112

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.
*/

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

        const int S = sum - root->val;
        if(!root->left && !root->right && 0==S)
        {
            return true;
        }

        bool L = hasPathSum(root->left, S);
        if(L) { return true; }

        return hasPathSum(root->right, S);
    }
};