LeetCode 114: Flatten Binary Tree to Linked List

/**
 * 114. Flatten Binary Tree to Linked List
 * 1. Time:O(n)  Space:O(logn)
 * 2. Time:O(n)  Space:O(1)
 * 3. Time:O(n)  Space:O(logn)
 */

// 1. Time:O(n)  Space:O(logn)
class Solution {
    public void flatten(TreeNode root) {
        if(root==null) return;
        flatten(root.left);
        flatten(root.right);
        if(root.left==null) return;
        TreeNode tmp = root.left;
        while(tmp.right!=null) tmp = tmp.right;
        tmp.right = root.right;
        root.right = root.left;
        root.left = null;
    }
}

// 2. Time:O(n)  Space:O(1)
class Solution {
    public void flatten(TreeNode root) {
        TreeNode cur = root;
        TreeNode prev = null;
        while(cur!=null){
            if(cur.left==null) 
                cur = cur.right;
            else{
                prev = cur.left;
                while(prev.right!=null)
                    prev = prev.right;
                prev.right = cur.right;
                cur.right = cur.left;
                cur.left = null;
            }
        }
    }
}

// 3. Time:O(n)  Space:O(logn)
class Solution {
    private TreeNode prev = null;
    public void flatten(TreeNode root) {
        if(root==null) return;
        flatten(root.right);
        flatten(root.left);
        root.right = prev;
        root.left = null;
        prev = root;
    }
}