[LeetCode] Implement Queue using Stacks
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, andis emptyoperations 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).
解题思路：
用两个栈来模拟队列。其中一个栈做缓存之用，另外一个栈只接受。push操作，直接放入接收之栈中。pop操作，若缓存栈存在数据，则从缓存栈中弹出。若缓存栈为空，则将接收栈的数据弹出并插入到缓存栈中，再从缓存栈中弹出。peek操作与pop操作类似。当且仅当两个栈都为空时，empty才返回true。
class Queue {
public:
// Push element x to the back of queue.
void push(int x) {
stacks[0].push(x);
}
// Removes the element from in front of queue.
void pop(void) {
if(stacks[1].empty()){
while(!stacks[0].empty()){
stacks[1].push(stacks[0].top());
stacks[0].pop();
}
}
stacks[1].pop();
}
// Get the front element.
int peek(void) {
if(stacks[1].empty()){
while(!stacks[0].empty()){
stacks[1].push(stacks[0].top());
stacks[0].pop();
}
}
return stacks[1].top();
}
// Return whether the queue is empty.
bool empty(void) {
return stacks[0].empty() && stacks[1].empty();
}
private:
stack<int> stacks[2];
};
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