232. Implement Queue using Stacks

Problem:

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.
  Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

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, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

思路：

使用两个栈，入栈和出栈。当元素插入队列时，将元素放入入栈。如果要删除或者返回队头元素，因为队头元素位于入栈的底部，所以需要将入栈中的元素依次放入出栈，则出栈中的元素顺序就和队列顺序相同了，后面插入的元素继续放入入栈，直到出栈中的元素都弹出时再重复上述操作。

Solution (C++):

stack<int> input, output;
/** Initialize your data structure here. */
MyQueue() {
    
}

/** Push element x to the back of queue. */
void push(int x) {
    input.push(x);
}

/** Removes the element from in front of queue and returns that element. */
int pop() {
    int x = peek();
    output.pop();
    return x;
}

/** Get the front element. */
int peek() {
    if (output.empty()) {
        while (!input.empty()) {
            output.push(input.top());
            input.pop();
        }
    }
    return output.top();
}

/** Returns whether the queue is empty. */
bool empty() {
    return input.empty() && output.empty();
}

性能：

Runtime: 0 ms  Memory Usage: 6.6 MB

思路：

Solution (C++):

性能：

Runtime: ms  Memory Usage: MB