Recover Binary Search Tree--leetcode难题讲解

Two elements of a binary search tree (BST) are swapped by mistake.
Recover the tree without changing its structure.
Note:
A solution using O(n) space is pretty straight forward. Could you devise a constant space solution?

https://leetcode.com/problems/recover-binary-search-tree/

二叉排序树中有两个节点被交换了，要求把树恢复成二叉排序树。

最简单的办法，中序遍历二叉树生成序列，然后对序列中排序错误的进行调整。最后再进行一次赋值操作，但这个不符合空间复杂度要求。

需要用两个指针记录错误的那两个元素，然后进行交换。

怎样找出错误的元素？遍历二叉排序树，正确时应该是从小到大，如果出现之前遍历的节点比当前大，则说明出现错误。所以我们需要一个pre指针来指向之前经过的节点。

如果只有一处不符合从小到大，则只用交换这一个地方。第二个指针记录第二个异常点。

Github repository: https://github.com/huashiyiqike/myleetcode

//JAVA CODE：
public class Solution {
    TreeNode first = null, second = null, pre = null;
     //first larger than follow, second smaller than pre
    public void helper(TreeNode root){
        if(root.left != null) helper(root.left);
        if(pre != null && root.val < pre.val){
            if(first == null)
                first = pre;
            second = root;
        }
        pre = root;
        if(root.right != null) helper(root.right);
    }
    public void recoverTree(TreeNode root) {
        helper(root);
        int tmp = first.val;
        first.val = second.val;
        second.val = tmp;
    }
}

//C++ CODE：
#include <iostream>
#include <cstdlib>
struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

class Solution {
    TreeNode *first = NULL, *second = NULL, *last = NULL;
public:
    void inorder(TreeNode* root){
        if(root->left != NULL){
            inorder(root->left);
        }
        if(last != NULL && root->val < last->val){
            if(first == NULL) first = last;
            second = root;
        }
        last = root;
        if(root->right != NULL) inorder(root->right);
    }
    void recoverTree(TreeNode* root) {
        if(root == NULL) return;
        inorder(root);
        if(first && second)
            std::swap(first->val, second->val);
    }
};

#PYTHON CODE：
class Solution:
    def inorderTravel(self, root, last):
        if root.left:
            last = self.inorderTravel(root.left, last)
        if last and last.val > root.val:
            if self.first is None:
                self.first = last
            self.second = root
        last = root
        if root.right:
            last = self.inorderTravel(root.right, last)
        return last

    def recoverTree(self, root):
        if root is None:
            return
        self.first, self.second = None, None
        self.inorderTravel(root, None)
        self.first.val, self.second.val = self.second.val, self.first.val