【LeetCode】232 & 225 - Implement Queue using Stacks & Implement Stack using Queues

232 - 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).

class Queue {
public:
// Push element x to the back of queue.
void push(int x) {
stk.push(x);
}

// Removes the element from in front of queue.
void pop(void) {
stack<int> temp;
while(stk.size()!=1){
int x=stk.top();
stk.pop();
temp.push(x);
}
stk.pop();
while(!temp.empty()){
int x=temp.top();
temp.pop();
stk.push(x);
}
}

// Get the front element.
int peek(void) {
stack<int> temp;
while(stk.size()!=1){
int x=stk.top();
stk.pop();
temp.push(x);
}
int x=stk.top();
while(!temp.empty()){
int x=temp.top();
temp.pop();
stk.push(x);
}
return x;
}

// Return whether the queue is empty.
bool empty(void) {
return stk.empty();
}
private:
stack<int> stk;
}

225 - Implement Stack using Queues

Implement the following operations of a stack using queues.

push(x) -- Push element x onto stack.

pop() -- Removes the element on top of the stack.

top() -- Get the top element.

empty() -- Return whether the stack is empty.

Notes:

You must use only standard operations of a queue -- which means only

push to back

,

peek/pop from front

,

size

, and

is empty

operations are valid.

Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.

You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

class Stack {
public:
// Push element x onto stack.
void push(int x) {
q.push(x);
}

// Removes the element on top of the stack.
void pop() {
queue<int> temp;
while(q.size()!=1){
int x=q.front();
q.pop();
temp.push(x);
}
q=temp;
}

// Get the top element.
int top() {
queue<int> temp;
while(q.size()!=1){
int x=q.front();
q.pop();
temp.push(x);
}
int x=q.front();
q=temp;
q.push(x);
return x;
}

// Return whether the stack is empty.
bool empty() {
return q.empty();
}
private:
queue<int> q;
};