【树】Flatten Binary Tree to Linked List（先序遍历）

题目：

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

思路：

按照树的先序遍历顺序把节点串联起来即可。

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {void} Do not return anything, modify root in-place instead.
 */
var flatten = function(root) {
    if(root==null){
        return;
    }
    var stack=[],pre=null;
    stack.push(root);
    
    while(stack.length!=0){
        var p=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;
};