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225 Implement Stack using Queues

OA: class MyStack { Queue<Integer> queue; /** Initialize your data structure here. */ public MyStack () { queue = new LinkedList<>(); } /** Push element x onto stack. */ public void push ( int x) { queue.offer(x); int k = queue.size() - 1 ; while (k > 0 ) { queue.offer(queue.poll()); k--; } } /** Removes the element on top of the stack and returns that element. */ public int pop () { return queue.poll(); } /** Get the top element. */ public int top () { return queue.peek(); } /** Returns whether the stack is empty. */ public boolean empty () { return queue.isEmpty(); } } oa是push一次就逆序排一次（相当于每次push一个之后都把新push进去的那个排到最前面，这样下次pop的时候就是pop的这一个。） ma在top和pop的时候才把最后一个调到前面来，而且每次都要重新排。（感觉其实差不多，要看这个这个stack是pop和top用得多还是push用得多） MA: class MyStack { ArrayDeque<Integer> ...
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232. Implement Queue using Stacks

MA: class MyQueue {     Stack<Integer> simQ=new Stack<Integer>();     Stack<Integer> buffer=new Stack<Integer>();     /** Initialize your data structure here. */     public MyQueue() {     }         /** Push element x to the back of queue. */     public void push(int x) {         simQ.push(x);     }         /** Removes the element from in front of queue and returns that element. */     public int pop() {         while(!simQ.empty()){             buffer.push(simQ.pop());         }         int temp=buffer.pop();         while(!buffer.empty()){             simQ.push(buffer.pop());         }         return temp;     } ...
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20. Valid Parentheses

OA: class Solution { public boolean isValid (String s) { char [] stack = new char [s.length()]; int head = 0 ; for ( char c : s.toCharArray()) { if (c == '(' ) { stack[head++] = c; } else if (c == '[' ) { stack[head++] = c; } else if (c == '{' ) { stack[head++] = c; } else if (c == ')' ) { if (head == 0 ) return false ; if (stack[--head] != '(' ) return false ; } else if (c == ']' ) { if (head == 0 ) return false ; if (stack[--head] != '[' ) return false ; } else if (c == '}' ) { if (head == 0 ) return false ; if (stack[--head] != '{' ) return false ; } } return head == 0 ; } } OA比MA快一倍。 String有个toCharArray()...
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