/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public boolean hasPathSum(TreeNode root, int sum) {
// corner case
if (root == null) {
return false;
}
// if the node is a leaf, it must has path sum,
// otherwise it depends on the left or right tree has path sum or not
if (root.left == null && root.right == null && root.val == sum) {
return true;
} else {
return hasPathSum(root.left, sum - root.val) || hasPathSum(root.right, sum - root.val);
}
}
}