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

Pre analysis

Will attack by problem in bottom up approach

Another Solution

var flatten = function(root) {
    let prev = null
    
    function moveToRight(node) {
        if(node == null) {
            return;
        }
        
        //post order
        moveToRight(node.right);
        moveToRight(node.left);
        
        //task
        node.left = null;
        node.right = prev;
        prev = node
    }
    
    moveToRight(root);
};