"Tree" Flatten Binary tree to Linked List (first-order traversal)

Topic:

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

Ideas:

The nodes are concatenated according to the sequence of the tree sequence traversal.

/** Definition for a binary tree node. * Function TreeNode (val) {* This.val = val; * This.left = This.right = NULL; * } *//** * @param {TreeNode} root * @return {void} does not return anything, modify root in-place instead.*/varFlatten =function(root) {if(root==NULL){        return; }    varStack=[],pre=NULL;        Stack.push (root);  while(stack.length!=0){        varp=Stack.pop (); if(pre!=NULL) {Pre.right=p; Pre.left=NULL        }        if(p.right) {Stack.push (p.right); }        if(p.left) {Stack.push (p.left); } Pre=p; } pre.left=NULL; Pre.right=NULL;};

"Tree" Flatten Binary tree to Linked List (first-order traversal)