99. Recover Binary Search Tree

/**
 * 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 {
public:
    void markVal(TreeNode* root, TreeNode* &prev, TreeNode* &first, TreeNode* &second)
    {
        if(root == NULL)    return;

        markVal(root -> left, prev, first, second);

        if(prev)
        {
            if(prev -> val > root -> val)
            {   
                if(first == NULL){
                    first = prev;
                }              
                second = root;
            }
        }

        prev = root;

        markVal(root -> right, prev, first, second);
    }

    void recoverTree(TreeNode* root) {
        TreeNode* prev = NULL;
        TreeNode* first = NULL;
        TreeNode* second = NULL;

        markVal(root, prev, first, second);

        int temp = first -> val;
        first -> val = second -> val;
        second -> val = temp;
    }
};