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

Note:

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

题意：给定一个二叉树，计算整个树的坡度。

思路 递归后序遍历

注意：一个二叉树的节点坡度定义为，该节点左子树的结点值之和和右子树结点值之和的差的绝对值。空结点的的坡度是 0 。题目要求出整个树的坡度，就是所有节点的坡度之和。递归后序遍历的代码如下：

class Solution {
   public:    int findTilt(TreeNode* root) {
           int tilt = 0;        postorder(root, tilt);        return tilt;    }    int postorder(TreeNode *root, int &tilt) {
    //返回子树的值之和        if (!root) return 0;        if (!root->left && !root->right) return root->val;         int lsum = postorder(root->left, tilt);        int rsum = postorder(root->right, tilt);        tilt += abs(lsum - rsum);        return root->val + lsum + rsum;    }};

效率较低：

执行用时：32 ms, 在所有 C++ 提交中击败了67.95% 的用户内存消耗：23.2 MB, 在所有 C++ 提交中击败了30.93% 的用户

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