LeetCode Implement Stack using Queues
2015-07-27 16:38
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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
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).
Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.
思路分析:这题和LeetCode Implement Queue using Stacks类似,思路也类似,用两个队列来模拟一个栈。具体做法是,入栈只需要放入Q1队列尾部。出栈时,把Q1除了最后一个元素之外的所有元素出队列,并且压入Q2队列尾部,然后从Q1中取出最后那个元素。注意要保证Q2为空,它是一个辅助队列,所以我们交换Q1和Q2。top和pop方法类似,除了取出Q1中最后那个元素后,再返回它前还要压入到Q2中去,保证这个元素不被丢失,因为我们只想看栈顶元素,并不想真正将它出栈。
AC Code
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 emptyoperations 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).
Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.
思路分析:这题和LeetCode Implement Queue using Stacks类似,思路也类似,用两个队列来模拟一个栈。具体做法是,入栈只需要放入Q1队列尾部。出栈时,把Q1除了最后一个元素之外的所有元素出队列,并且压入Q2队列尾部,然后从Q1中取出最后那个元素。注意要保证Q2为空,它是一个辅助队列,所以我们交换Q1和Q2。top和pop方法类似,除了取出Q1中最后那个元素后,再返回它前还要压入到Q2中去,保证这个元素不被丢失,因为我们只想看栈顶元素,并不想真正将它出栈。
AC Code
class MyStack { //Queue LinkedList<Integer> q1 = new LinkedList<Integer>(); LinkedList<Integer> q2 = new LinkedList<Integer>(); // Push element x onto stack. public void push(int x) { q1.add(x); } // Removes the element on top of the stack. public void pop() { //peek(); poll(); while(q1.size() > 1){ q2.add(q1.poll()); } q1.poll(); //switch LinkedList<Integer> tem = q1; q1 = q2; q2 = tem; } // Get the top element. public int top() { //peek(); poll(); while(q1.size() > 1){ q2.add(q1.poll()); } int res = q1.peek(); q2.add(q1.poll()); //switch LinkedList<Integer> tem = q1; q1 = q2; q2 = tem; return res; } // Return whether the stack is empty. public boolean empty() { return q1.isEmpty(); } }
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