155. Min Stack

这题居然想了半天没想出来……
讲解：https://www.cnblogs.com/grandyang/p/4091064.html
关于stack.pop()返回值类型不能直接==的一些参考：
关于Integer的==和equals：
https://stackoverflow.com/questions/3637936/java-integer-equals-vs

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225 Implement Stack using Queues

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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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