225. 用队列实现栈

解题思路

1. push操作是直接把元素放入队列里面
2. pop操作时把队列头的元素放入到队列尾，重复队列元素个数减一次
3. top操作就是pop加push操作

代码实现

typedef struct {
    int q[1001];
    int l;
    int r;
} MyStack;


MyStack* myStackCreate() {  
    MyStack* ans = (MyStack*)malloc(sizeof(MyStack));

    ans -> r = ans -> l = 0;

    return ans;
}

void myStackPush(MyStack* obj, int x) {
    obj -> q[obj -> r ++] = x;
}

int myStackPop(MyStack* obj) {
    for (int i = 0; i < obj -> r - obj -> l - 1; i ++) {
       myStackPush(obj, obj -> q[obj -> l ++]);
    }

    return obj -> q[obj -> l++];
}

int myStackTop(MyStack* obj) {
    int ans = myStackPop(obj);

    myStackPush(obj, ans);

    return ans;
}

bool myStackEmpty(MyStack* obj) {
    return obj -> l == obj -> r;
}

void myStackFree(MyStack* obj) {
    obj -> l = obj -> r = 0;
}

/**
 * Your MyStack struct will be instantiated and called as such:
 * MyStack* obj = myStackCreate();
 * myStackPush(obj, x);
 
 * int param_2 = myStackPop(obj);
 
 * int param_3 = myStackTop(obj);
 
 * bool param_4 = myStackEmpty(obj);
 
 * myStackFree(obj);
*/