99. 恢复二叉搜索树

题解：

链接：https://leetcode-cn.com/problems/recover-binary-search-tree/solution/tu-jie-hui-fu-yi-ge-er-cha-sou-suo-shu-by-hyj8/

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
private:
    // 记录中序遍历前一个节点
    TreeNode* prev = nullptr;
    // 记录两个交换的节点
    TreeNode* x = nullptr;
    TreeNode* y = nullptr;
public:
    void recoverTree(TreeNode* root) {
        inorder(root);
        if(x && y) {
            swap(x->val, y->val);
        }
    }
    void inorder(TreeNode* root) {
        if(!root) return;
        inorder(root->left);
        // 判断前面的值是否大于后面的值
        if(prev && prev->val > root->val) {
            y = root;
            // 第二次，本次不修改
            if(x) {
                return;
            }
            x = prev;
        }
        // 更新prev节点
        prev = root;
        inorder(root->right);
    }
};