[leetcode]155. Min Stack最小栈

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.

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.

题意：

请设计一个栈结构，支持 push、pop、top以及getMin操作，且每个操作的时间复杂度都是 O(1)。

思路：

维护一个单调栈minStack

code

class MinStack {

    /** initialize your data structure here. */
    Stack<Integer> s ;
    Stack<Integer> minStack ;
    
    public MinStack() {
            s = new Stack<>();
            minStack = new Stack<>();
    }
    
    public void push(int x) {
        s.push(x);
        int minValue = minStack.isEmpty() ? x: Math.min(minStack.peek(), x);
        minStack.push(minValue);
        
    }
    
    public void pop() {
        s.pop();
        minStack.pop();
    }
    
    public int top() {
        return s.peek();
        
        
    }
    
    public int getMin() {
        return minStack.peek();
    }
}