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.

              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：使用先序遍历，不过本人在压栈的时候改变了节点的值。

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
	public boolean hasPathSum(TreeNode root, int sum) {
	    if(root == null ) return false;
		Stack<TreeNode> s = new Stack<TreeNode>();
		s.push(root);
		while (!s.isEmpty()) {
			TreeNode temp = s.peek();
			s.pop();
			if (temp.right != null) {
				temp.right.val += temp.val;
				s.push(temp.right);
			}
			if (temp.left != null) {
				temp.left.val += temp.val;
				s.push(temp.left);
			}
			if (temp.left == null && temp.right == null && temp.val == sum)
				return true;
		}
		return false;
	}
}

LeetCode 112 Path Sum