Flatten Binary Tree to Linked List

版本递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public void flatten(TreeNode root) {
        if(root == null) return;
        
        if(root.left != null) flatten(root.left);
        if(root.right != null) flatten(root.right);
        
        TreeNode tmp = root.right;
        root.right = root.left;
        root.left = null;
        while(root.right != null)
            root = root.right;
        root.right = tmp;
        
    }
}

非递归版本 o(n)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public void flatten(TreeNode root) {
        if(root ==null) return;
        TreeNode cur = root;
        while(cur != null){
            if(cur.left != null){
                TreeNode p = cur.left;
                while(p.right != null) p = p.right;
                p.right = cur.right;
                cur.right = cur.left;
                cur.left = null;
            }
            
            cur = cur.right;
        }
        
    }
}