[jobdu]用两个栈实现队列

思路比较简单。就是当要pop的时候，如果s2为空，才把s1的转过来。总之就是区分一下此时s2为空和非空的情况。 http://ac.jobdu.com/problem.php?pid=1512

#include <cstring>
#include <cstdio>
#include <stack>
using namespace std;

class Queue {
public:
    void push(int val) {
        s1.push(val);
    }
    int pop() {
        if (s2.empty())
        {
            while(!s1.empty())
            {
                int tmp = s1.top();
                s1.pop();
                s2.push(tmp);
            }
        }
        if (!s2.empty()) {
            int tmp = s2.top();
            s2.pop();
            return tmp;
        }
        else {
            return -1;
        }
    }
private:
    stack<int> s1, s2;
};

int main() {
    Queue Q;
    int n; scanf("%d", &n);
    char flag[20]; int val;
    for (int i=0; i<n; ++i) {
        scanf("%s", flag);
        if (strcmp("PUSH", flag) == 0) {
            scanf("%d", &val);
            Q.push(val);
        } else if (strcmp("POP", flag) == 0) {
            printf("%d\n", Q.pop());
        }
    }
    return 0;
}