【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);
}
}