563. Binary Tree Tilt

QUESTION:

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.

EXPLANATION:

一开始的时候题目看错了，以为只是左右两个数的减然后再累加就可以了。但是发现提交上去的结果不对，然后再重新查看了一下题目，才发现是左右子树的和的 差的绝对值。

那么也挺简单的：

1.如果root是null就返回0

2.左树的值，右树的值

3.将左右树差取绝对值，加入结果中

4.返回的左右两树的和加上本身的值，作为上一个结果

SOLUTION:

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