算法作业_24(2017.5.23第十四周)

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

刚开始总是理解错题意，导致总是错误。这道题其实是要求 求出各个节点左右子树的差的绝对值，将这些绝对值求和并返回

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