663. Equal Tree Partition

package LeetCode_663

import main.TreeNode
import java.util.*

/**
 * 663. Equal Tree Partition
 * (locked by leetcode)
 * https://www.lintcode.com/problem/equal-tree-partition/description
 *
 * Given a binary tree with n nodes, your task is to check if it's possible to partition the tree to two trees which have the equal sum of values after removing exactly one edge on the original tree.

Example 1:
Input:
   5
  / \
 10 10
   /  \
  2   3
Output: True

Explanation:
  5
 /
10
Sum: 15

  10
 /  \
2    3
Sum: 15
 * */
class Solution {
    /*
    * solution: DFS,
    * use stack to record the sum of every subtree
    * Time complexity:O(n), Space complexity:O(n)
    * */

    private val stack = Stack<Int>()

    fun checkEqualTree(root: TreeNode?): Boolean {
        if (root == null) {
            return true
        }
        sum(root)
        val total = stack.pop()
        if (total % 2 == 0) {
            for (item in stack) {
                if (item == total / 2) {
                    return true
                }
            }
        }
        return false
    }

    private fun sum(root: TreeNode?): Int {
        if (root == null) {
            return 0
        }
        val sum = sum(root.left) + sum(root.right) + root.`val`
        stack.push(sum)
        return stack.peek()
    }
}