binary-tree-preorder-traversal 使用栈来前序遍历二叉树

不用递归，那就使用栈来遍历即可

import java.util.ArrayList;
import java.util.Stack;

public class Solution {
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        ArrayList<Integer> list = new ArrayList<Integer>();
        if(root != null){
            stack.push(root);
        }
        while(stack.size() > 0){
            if(stack.peek() != null){
                list.add(stack.peek().val);
                stack.push(stack.peek().left);
            }else{
                stack.pop();
                if(stack.size() > 0 && stack.peek() != null){
                    stack.push(stack.pop().right);
                }
            }

        }
        return list;

    }
}

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */