两个栈实现一个队列

public class Code_03_StackAndQueueConvert {

	public static class TwoStacksQueue {
		private Stack<Integer> stackPush;
		private Stack<Integer> stackPop;

		public TwoStacksQueue() {
			stackPush = new Stack<Integer>();
			stackPop = new Stack<Integer>();
		}

		public void push(int pushInt) {
		// 直接压入
			stackPush.push(pushInt);
		}

		public int poll() {
			if (stackPop.empty() && stackPush.empty()) {
				throw new RuntimeException("Queue is empty!");
			} else if (stackPop.empty()) {
				while (!stackPush.empty()) {
				//  stackPush不为空的时候，才能往stackPop中加入
					stackPop.push(stackPush.pop());
				}
			}
			return stackPop.pop();
		}

		public int peek() {
			if (stackPop.empty() && stackPush.empty()) {
				throw new RuntimeException("Queue is empty!");
			} else if (stackPop.empty()) {
				while (!stackPush.empty()) {
					stackPop.push(stackPush.pop());
				}
			}
			return stackPop.peek();
		}
	}

stackPush和stacPop必须遵守的两条

1、如果Pop栈中有数，则不能把Push栈中的数加入，否则会成乱序

2、如果要将Push栈中的数压入Pop中，一次必须压完。