Implement Queue using Stacks
Description
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
, andis 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).
Solution
class MyQueue {
public:
/** Initialize your data structure here. */
MyQueue() {
}
/** Push element x to the back of queue. */
void push(int x) {
st.push(x);
}
/** Removes the element from in front of queue and returns that element. */
int pop() {
if (rst.empty())
transfer(rst, st);
int val = rst.top();
rst.pop();
return val;
}
/** Get the front element. */
int peek() {
if (rst.empty())
transfer(rst, st);
return rst.top();
}
void transfer(stack<int> &s1, stack<int> &s2) {
while (!s2.empty()) {
s1.push(s2.top());
s2.pop();
}
}
/** Returns whether the queue is empty. */
bool empty() {
return st.empty() && rst.empty();
}
stack<int> st, rst;
};
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