[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.

简单的树的遍历的题目，注意当输入为null的时候返回应该是false这一corner case.

public boolean hasPathSum(TreeNode root, int sum) {
        if(root == null) return false;
        if(root.left == null && root.right == null) return (sum - root.val) == 0;
        else if(root.left != null && root.right == null) return hasPathSum(root.left, sum - root.val);
        else if(root.left == null && root.right != null) return hasPathSum(root.right, sum - root.val);
        else
            return hasPathSum(root.left, sum - root.val) || hasPathSum(root.right, sum - root.val);
    }