[leetCode] Recover Binary Search Tree

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?

 

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

 

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 Tree Depth-first Search

 

 

思路：

O(n) 空间的解法是，开一个指针数组，中序遍历，将节点指针依次存放到数组里，然后寻找两
处逆向的位置，先从前往后找第一个逆序的位置，然后从后往前找第二个逆序的位置，交换这两个
指针的值。
中序遍历一般需要用到栈，空间也是 O(n) 的，如何才能不使用栈？Morris 中序遍历。

 

方法一：中序遍历

class Solution {
    public:
        void recoverTree(TreeNode *root) {
            vector<TreeNode*> result;
            stack<TreeNode*> st;

            TreeNode* p = root;

            // inorder traverse
            while(p != NULL || st.size() != 0)
            {
                while(p != NULL)
                {
                    st.push(p);
                    p = p->left;
                }

                if(!st.empty())
                {
                    p = st.top();
                    st.pop();
                    result.push_back(p);
                    p = p->right;
                }
            }

            TreeNode* r1 = NULL, *r2 = NULL;
            for(int i = 0; i < result.size()-1; i++)
            {
                if(result[i]->val  > result[i+1]->val)
                {
                    r1 = result[i];
                    break;
                }
            }

            for(int i = result.size()-1; i >= 1; i--)
            {
                if(result[i]->val < result[i-1]->val)
                {
                    r2 = result[i];
                    break;
                }
            }
            //swap r1 and r2's value

            int tmp = r1->val;
            r1->val = r2->val;
            r2->val = tmp;
        }
};