155. 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.

分析：使用一个stack，维持一个全局变量min，stack中存储的是当前的值和最小值的差。

public class MinStack {    long min;    Stack<Long> stack;    /** initialize your data structure here. */    public MinStack() {        stack = new Stack<Long>();    }        public void push(int x) {        if(stack.isEmpty()){            stack.push(0l);            min=x;        }else{            stack.push(x-min);            if(x<=min){                min=x;            }        }    }        public void pop() {        if(stack.isEmpty())            return;        if(stack.peek()<=0){            min=min-stack.peek();        }        stack.pop();    }        public int top() {        if(stack.peek()<=0){            return (int)min;        }else            return (int)(stack.peek()+min);    }        public int getMin() {        return (int)min;    }}/** * 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(); */