L114. Flatten Binary Tree to Linked List

recursion

Problem:

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

For example, given the following tree:

    1
   / \
  2   5
 / \   \
3   4   6

The flattened tree should look like:

1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

Solution:

• make all left as null

 public void flatten(TreeNode root) {
    if(root == null) return;
    TreeNode[] pre = new TreeNode[1];
        
    helper(root, pre);
}
private void helper(TreeNode root, TreeNode[] pre){
    if(root == null) return;
    
    helper(root.right, pre);
    helper(root.left, pre);
    
    root.right = pre[0];
    root.left = null;
    pre[0] = root;
}