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.

Solution：Post-order 分治

思路：
Time Complexity: O(N) Space Complexity: O(N) 递归缓存

Solution Code：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    int result = 0;
    
    public int findTilt(TreeNode root) {
        postOrder(root);
        return result;
    }
    
    private int postOrder(TreeNode root) {
        if (root == null) return 0;
        
        int left = postOrder(root.left);
        int right = postOrder(root.right);
        
        result += Math.abs(left - right);
        
        return left + right + root.val;
    }
}