剑指offer 09.用两个栈实现队列

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剑指offer 09.用两个栈实现队列

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链接：https://leetcode.cn/problems/yong-liang-ge-zhan-shi-xian-dui-lie-lcof/
1.暴力解法

class CQueue {

   private  Stack inStack,outStack;


    public CQueue() {
        if(null==inStack){
            inStack=new Stack<>();
        }
        if(null==outStack){
            outStack=new Stack<>();
        }
    }
    
    public void appendTail(int value) {
        inStack.push(value);
    }
    
    public int deleteHead() {
        int result=-1;
        //判空
        if(inStack.empty()){
            return result;
        }

        outStack.clear();
        
        //将inStack的元素全都pop到outStack中
        while(!inStack.empty()){
            outStack.push(inStack.pop());
        }
        //此时outStack的第一个元素就是需要删除的元素，将该元素pop出来
        result=outStack.pop();
        //再将outStack的元素pop回inStack中
        while(!outStack.empty()){
            inStack.push(outStack.pop());
        }
        return result;
        
    }
}

/**
 * Your CQueue object will be instantiated and called as such:
 * CQueue obj = new CQueue();
 * obj.appendTail(value);
 * int param_2 = obj.deleteHead();
 */

2.详情看注释

class CQueue {

   private  Stack inStack,outStack;


    public CQueue() {
        if(null==inStack){
            inStack=new Stack<>();
        }
        if(null==outStack){
            outStack=new Stack<>();
        }
    }
    
    public void appendTail(int value) {
        inStack.push(value);
    }
    
    public int deleteHead() {
        //outStack 里有元素则直接返回
        if(!outStack.empty()){
            return outStack.pop();
        }
     
        //判空
        if(inStack.empty()){
            return -1;
        }

        
        //将inStack的元素全都pop到outStack中
        //此时inStack为空了，也就意味着outStack是倒叙放了最后几个元素，下一次deleteHead时直接outStack.pop()即可
        while(!inStack.empty()){
            outStack.push(inStack.pop());
        }
      return outStack.pop();
        
    }
}

/**
 * Your CQueue object will be instantiated and called as such:
 * CQueue obj = new CQueue();
 * obj.appendTail(value);
 * int param_2 = obj.deleteHead();
 */