563. Binary Tree Tilt

Given a binary tree, return the tilt of the whole tree.

The tilt of a tree node is defined as the absolute difference between the sum of all left subtree node values and the sum of all right subtree node values. Null node has tilt 0.

The tilt of the whole tree is defined as the sum of all nodes' tilt.

Example:

Input: 
         1
       /   
      2     3
Output: 1
Explanation: 
Tilt of node 2 : 0
Tilt of node 3 : 0
Tilt of node 1 : |2-3| = 1
Tilt of binary tree : 0 + 0 + 1 = 1

Note:

1. The sum of node values in any subtree won't exceed the range of 32-bit integer.
2. All the tilt values won't exceed the range of 32-bit integer.

class Solution {
    int res = 0;
    public int findTilt(TreeNode root) {
        helper(root);
        return res;
    }
    public int helper(TreeNode root) {
        if(root == null) return 0;
        int left = helper(root.left);
        int right = helper(root.right);
        res += Math.abs(left - right);
        return left + right + root.val;
    }
}

哎 还是不能一下做出来tree的题。

https://leetcode.com/problems/binary-tree-tilt/discuss/102334/Java-Solution-post-order-traversal

有老哥总结的好：

recursive method要注意： 前中后序？返回什么东西？返回的值要来干嘛？返回后在当前level干嘛？

而且有时候，返回的东西只是作为中间值，不影响result（result此时是全局变量）。