155. Min Stack

1.描述

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.

2.分析

用两个栈，一个用来正常存储，另一个用来存储当前状态下的最小值。

3.代码

class MinStack {
private:
    stack s1,s2;
    
public:
    /** initialize your data structure here. */
    MinStack() {
        
    }
    
    void push(int x) {
        s1.push(x);
        if (s2.empty()) {
            s2.push(x);
        } else {
            s2.push(s2.top() < x ? s2.top() : x);
        }
    }
    
    void pop() {
        if (s1.empty() || s2.empty()) exit(-1);
        s1.pop();
        s2.pop();
    }
    
    int top() {
        if (s1.empty() || s2.empty()) exit(-1);
        return s1.top();
    }
    
    int getMin() {
        if (s1.empty() || s2.empty()) exit(-1);
        return s2.top();
    }
};

/**
 * 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();
 */