99. 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?

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constant space resolution: Morris Traversal or Morris Threading Traversal.

http://www.cnblogs.com/AnnieKim/archive/2013/06/15/morristraversal.html

也是用中序遍历的思想，找那个元素不是递增。Morris traversal用threaded binary tree进行中序遍历，只要Ｏ(1)的空间复杂度。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void recoverTree(TreeNode root) {
        TreeNode cur = root, prev = null, n1 = null, n2 = null, last = null;
        while (cur != null) {
            prev = cur.left;
            if (prev == null) { // visit current node
                if (last != null && cur.val < last.val) {
                    if (n1 == null) {
                        n1 = last;
                        n2 = cur;
                    } else {
                        n2 = cur;
                    }
                }
                last = cur;
                cur = cur.right; // jump to its next
                continue;
            }
            while (true) {
                if (prev.right == null) {
                    prev.right = cur;
                    cur = cur.left;
                    break;
                } else if (prev.right == cur) { // visit current node
                    prev.right = null;
                    if (cur.val < last.val) {
                        if (n1 == null) {
                            n1 = last;
                            n2 = cur;
                        } else {
                            n2 = cur;
                        }
                    }
                    last = cur;
                    cur = cur.right;
                    break;
                }
                prev = prev.right;
            }
        }
        int tmp = n1.val;
        n1.val = n2.val;
        n2.val = tmp;
    }
}

 

只会straight forward的Ｏ（n)解法：ＢＳＴ的中序遍历应该是递增的，对给的树中序遍历，如果碰到递减的元素，记下来，说明是被交换过的。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void recoverTree(TreeNode root) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode n1 = null, n2 = null, n3 = null, cur = root, prev = null, tmp = null;
        while (cur != null) {
            stack.push(cur);
            cur = cur.left;
        }
        while (!stack.empty()) {
            cur = stack.pop();
            if (prev != null && cur.val < prev.val) {
                if (n1 == null) {
                    n1 = prev;
                    n2 = cur;
                } else {
                    n2 = cur;
                    break;
                }
            }
            prev = cur;
            tmp = cur.right;
            while (tmp != null) {
                stack.push(tmp);
                tmp = tmp.left;
            }
        }
        // should find the parent of n1 and n2 instead of swapping values？
        int v1 = n1.val;
        n1.val = n2.val;
        n2.val = v1;
    }
}