LeetCode OJ 99. Recover Binary Search Tree

Posted 2020-08-18

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?

 

 

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解答：

这道题其实就是利用有错位时，中序遍历过程中会出现前后两个节点值的大小关系不符合BST性质的情况出现，这样只要把两个点都标记下来就好了……不过这里要注意1，2，3，4，5和1，3，2，4，5这样的错误只会在遍历中只会遇到一次错误情况，而1，2，3，4，5和3，2，1，4，5这样的错误在遍历中就会产生两次错误情况，所以在每一次遇到错误情况时就要标记两个点，如果有第二次遇到错误情况只要更新其中的一个标记就好了，当然要注意null节点……这个是用Java写的，用C写我不会啊TAT

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {  
    TreeNode tmp = null, mis_node_1 = null, mis_node_2;  
      
    void find_mistake(TreeNode root) {  
        if(root==null)   
            return;  
        find_mistake(root.left); 
        if(tmp != null&&root.val < tmp.val){  
            if(null == mis_node_1)  
                mis_node_1 = tmp;  
            mis_node_2 = root;  
        }  
        tmp = root;  
        find_mistake(root.right);  
    }  
    public void recoverTree(TreeNode root) {    
        find_mistake(root);  
        if(null != mis_node_1){  
            int tmp_val = mis_node_1.val;  
            mis_node_1.val = mis_node_2.val;  
            mis_node_2.val = tmp_val;  
        }  
    }  
}