Flatten Binary Tree to Linked List

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

Example

For example, given the following tree:

    1
   / \
  2   5
 / \   \
3   4   6

The flattened tree should look like:

1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

Note

都是在右边，所以先处理右侧

Code

class Solution {
    public void flatten(TreeNode root) {
        if (root == null) return;
        Stack<TreeNode> stack = new Stack<>();
        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;
        }
    }

    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;
    }

}