114. Flatten Binary Tree to Linked List

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
非递归

  public void flatten(TreeNode root) {
        if(root == null) return;
        if(root.right == null && root.left == null) return;
        LinkedList<TreeNode> stack = new LinkedList<TreeNode>();
        TreeNode tmp = root;
        if(root.right != null) stack.push(root.right);
        if(root.left != null) stack.push(root.left);
        while(!stack.isEmpty()){
            TreeNode cur = stack.pop();
            tmp.right = cur;
            tmp.left = null;
            tmp = tmp.right;
            if(cur.right != null) stack.push(cur.right);
            if(cur.left != null) stack.push(cur.left);
        }
    }

递归

 TreeNode pre;
    public void flatten(TreeNode root) {
        if(root == null) return;
        flatten(root.right);
        flatten(root.left);
        root.right = pre;
        root.left = null;
        pre = root;
    }