LeetCode—面试题：栈排序（辅助栈）

栈排序（中等）

2020年10月14日

题目来源：力扣

解题
辅助栈来存放主栈比push的值大的元素

class SortedStack {
    
    
    private Stack<Integer> st1;
    private Stack<Integer> st2;
    public SortedStack() {
    
    
        st1=new Stack<>();
        st2=new Stack<>();
    }
    
    public void push(int val) {
    
    
        if(st1.empty())
            st1.push(val);
        else{
    
    
            while(!st1.empty() && st1.peek()<val){
    
    
                st2.push(st1.pop());
            }
            st1.push(val);
            while(!st2.empty()){
    
    
                st1.push(st2.pop());
            }
        }
    }
    
    public void pop() {
    
    
        if(!st1.empty())
            st1.pop();
    }
    
    public int peek() {
    
    
        if(!st1.empty())
            return st1.peek();
        return -1;
    }
    
    public boolean isEmpty() {
    
    
        return st1.empty();
    }
}

/**
 * Your SortedStack object will be instantiated and called as such:
 * SortedStack obj = new SortedStack();
 * obj.push(val);
 * obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.isEmpty();
 */