【leetcode】114. Flatten Binary Tree to Linked List【java】

Given a binary tree, flatten it to a linked list in-place.

For example,
Given

         1        / \       2   5      / \   \     3   4   6

The flattened tree should look like:

   1    \     2      \       3        \         4          \           5            \             6

click to show hints.

Hints:

If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } *///DFS:使用栈/*public class Solution {    public void flatten(TreeNode root) {        if (root == null) {            return;        }        Stack<TreeNode> stack = new Stack<TreeNode>();        stack.push(root);        while (!stack.isEmpty()) {            TreeNode cur = stack.pop();             if (cur.right != null) {                stack.push(cur.right);            }            if (cur.left != null) {                stack.push(cur.left);            }            if (!stack.isEmpty()) {                cur.right = stack.peek();            }            cur.left = null;        }    }}*/public class Solution {    private TreeNode lastNode = null;    public void flatten(TreeNode root) {        if (root == null) {            return;        }        if (lastNode != null) {            lastNode.left = null;            lastNode.right = root;        }        lastNode = root;        TreeNode left = root.left;        TreeNode right = root.right;        flatten(left);        flatten(right);    }}