232. Implement Queue using Stacks
Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).
Implement the MyQueue class:
- void push(int x) Pushes element x to the back of the queue.
- int pop() Removes the element from the front of the queue and returns it.
- int peek() Returns the element at the front of the queue.
- boolean empty() Returns true if the queue is empty, false otherwise.
Notes:
- You must use only standard operations of a stack, which means only push to top, peek/pop from top, size, and is empty operations are valid.
- Depending on your language, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack’s standard operations.
Example 1:
Input:
[“MyQueue”, “push”, “push”, “peek”, “pop”, “empty”]
[[], [1], [2], [], [], []]
Output:
[null, null, null, 1, 1, false]
Explanation:
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false
Constraints:
- 1 <= x <= 9
- At most 100 calls will be made to push, pop, peek, and empty.
- All the calls to pop and peek are valid.
From: LeetCode
Link: 232. Implement Queue using Stacks
Solution:
Ideas:
- Stack operations: These are standard stack operations (push, pop, peek, empty).
- Queue operations: push always pushes onto stack1. For pop and peek, if stack2 is empty, elements from stack1 are moved to stack2 to maintain FIFO order.
- Memory management: The myQueueFree function frees the memory allocated for the queue and its stacks.
Code:
#define MAX_SIZE 100
typedef struct {
int data[MAX_SIZE];
int top;
} Stack;
typedef struct {
Stack *stack1;
Stack *stack2;
} MyQueue;
Stack* createStack() {
Stack *stack = (Stack*)malloc(sizeof(Stack));
stack->top = -1;
return stack;
}
MyQueue* myQueueCreate() {
MyQueue *queue = (MyQueue*)malloc(sizeof(MyQueue));
queue->stack1 = createStack();
queue->stack2 = createStack();
return queue;
}
void stackPush(Stack *stack, int x) {
if (stack->top < MAX_SIZE - 1) {
stack->data[++(stack->top)] = x;
}
}
int stackPop(Stack *stack) {
if (stack->top >= 0) {
return stack->data[(stack->top)--];
}
return -1; // Stack is empty
}
int stackPeek(Stack *stack) {
if (stack->top >= 0) {
return stack->data[stack->top];
}
return -1; // Stack is empty
}
bool stackEmpty(Stack *stack) {
return stack->top == -1;
}
void myQueuePush(MyQueue* obj, int x) {
stackPush(obj->stack1, x);
}
int myQueuePop(MyQueue* obj) {
if (stackEmpty(obj->stack2)) {
while (!stackEmpty(obj->stack1)) {
stackPush(obj->stack2, stackPop(obj->stack1));
}
}
return stackPop(obj->stack2);
}
int myQueuePeek(MyQueue* obj) {
if (stackEmpty(obj->stack2)) {
while (!stackEmpty(obj->stack1)) {
stackPush(obj->stack2, stackPop(obj->stack1));
}
}
return stackPeek(obj->stack2);
}
bool myQueueEmpty(MyQueue* obj) {
return stackEmpty(obj->stack1) && stackEmpty(obj->stack2);
}
void myQueueFree(MyQueue* obj) {
free(obj->stack1);
free(obj->stack2);
free(obj);
}
/**
* Your MyQueue struct will be instantiated and called as such:
* MyQueue* obj = myQueueCreate();
* myQueuePush(obj, x);
* int param_2 = myQueuePop(obj);
* int param_3 = myQueuePeek(obj);
* bool param_4 = myQueueEmpty(obj);
* myQueueFree(obj);
*/