【LeetCode】232. Implement Queue using Stacks

LeetCode-232

Links:https://leetcode.com/problems/implement-queue-using-stacks/

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.

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：解决方法多种，我这里采用使用两个Stack的方法，代码如下：

class Queue {
public:
    stack<int>st;
    stack<int>temp;
    // Push element x to the back of queue.
    void push(int x) {
        st.push(x);
    }

    // Removes the element from in front of queue.
    void pop(void) {
        while(!st.empty())
        {
            int val = st.top();
            temp.push(val);
            st.pop();
        }
        temp.pop();
        while(!temp.empty())
        {
            int val = temp.top();
            st.push(val);
            temp.pop();
        }
    }

    // Get the front element.
    int peek(void) {
        while(!st.empty())
        {
            int val = st.top();
            temp.push(val);
            st.pop();
        }
        int ans = temp.top();
        while(!temp.empty())
        {
            int val = temp.top();
            st.push(val);
            temp.pop();
        }
        return ans;
    }

    // Return whether the queue is empty.
    bool empty(void) {
        return st.empty();
    }
};

class Queue {

public:

stack<int>st;

stack<int>temp;

// Push element x to the back of queue.

void push(int x) {

st.push(x);

}

// Removes the element from in front of queue.

void pop(void) {

while(!st.empty())

{

int val = st.top();

temp.push(val);

st.pop();

}

temp.pop();

while(!temp.empty())

{

int val = temp.top();

st.push(val);

temp.pop();

}

// Get the front element.

int peek(void) {

while(!st.empty())

{

int val = st.top();

temp.push(val);

st.pop();

}

int ans = temp.top();

while(!temp.empty())

{

int val = temp.top();

st.push(val);

temp.pop();

}

return ans;

}

// Return whether the queue is empty.

bool empty(void) {

return st.empty();

}

};

【LeetCode】232. Implement Queue using Stacks

Tagged on: LeetCode