112. Path Sum

/**
 * 112. Path Sum
 * https://leetcode.com/problems/path-sum/description/
 *
 * 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.

Note: A leaf is a node with no children.

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.
 * */
import java.util.*

class Solution {
    fun hasPathSum(root_: TreeNode?, sum: Int): Boolean {
       //preorder, root->left->right
        var root = root_
        val stack = Stack<TreeNode>()
        stack.push(root)
        while (!stack.isEmpty()){
            root = stack.pop()
            if (root==null){
                continue
            }
            if (root.left==null && root.right==null){
                //node is leaf
                if (root.`val`==sum){
                    return true
                }
            }
            //child's val add up with it parent's val
            if (root.left!=null){
                root.left.`val`+=root.`val`
                stack.push(root.left)
            }
            if (root.right!=null){
                root.right.`val`+=root.`val`
                stack.push(root.right)
            }
        }
        return false
    }
}