新手练手感--part1.1 栈与队列之用一个栈实现另一个栈的排序

题目：

　　给你一个栈，里面若干元素，只能在额外申请一个栈，请实现栈顶到栈底的排序问题

思路：

（1）一个栈stack，另一个栈help

（2）int a = stack.pop(),help.empty||a > help.peek,直接helo.put(a)

（3）a < help.peek()，则help.pop, stack.push,直到a >= help.peek()

public class TestCode {


    public static void main(String[] args) {
        //存放栈数据
        Stack<Integer> stack = new Stack<Integer>();
        for(int i = 0; i < 10; i++) {
            stack.push((int)(Math.random() * 10));
        }
        Stack<Integer> help = new Stack<Integer>();
        while(!stack.isEmpty()) {
            int num = stack.pop();
            if(help.isEmpty() ||num >= help.peek() ) {
                help.push(num);
            } else {
                //在help栈顶比num大时，弹出help中元素，压入stack中
                while(!help.isEmpty() && help.peek() > num) {
                    stack.push(help.pop());
                }
                help.push(num);
            }
        }

        while(!help.isEmpty()) {
            stack.push(help.pop());
            System.out.print("  " + stack.peek());
        }
    }

}