LeetCode(Design)Min Stack

Min Stack

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

• push(x) – Push element x onto stack.
• pop() – Removes the element on top of the stack.
• top() – Get the top element.
• getMin() – Retrieve the minimum element in the stack.

Example:

MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   --> Returns -3.
minStack.pop();
minStack.top();      --> Returns 0.
minStack.getMin();   --> Returns -2.

思路：可以考虑用ArrayList来实现stack的功能，当然也可以直接利用stack。

代码如下：

class MinStack {
    private ArrayList<Integer> minStack;
    /** initialize your data structure here. */
    public MinStack() {
        minStack=new ArrayList<Integer>();
    }
    
    public void push(int x) {
        minStack.add(x);
    }
    
    public void pop() {
        if(minStack==null){
           return;
        }
        minStack.remove(minStack.size()-1);
    }
    
    public int top() {
       if(minStack==null){
            return 0;
        }
        return minStack.get(minStack.size()-1);
    }
    
    public int getMin() {
        if(minStack==null){
            return 0;
        }
        ArrayList<Integer> temp=new ArrayList<Integer>(minStack);
        temp.sort(null);
        return temp.get(0);
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(x);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */