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.

思路：用两个栈，其中一个栈用来维护最小值序列，栈顶为当前序列最小值。

class MinStack {
public:
    /** initialize your data structure here. */
    stack<int> s;
    stack<int> ms;
    MinStack() {
        while(!s.empty()) s.pop();
        while(!ms.empty()) ms.pop();
    }
    void push(int x) {
        s.push(x);
        if (ms.empty() || x <= ms.top()) ms.push(x);
    }
  
    void pop() {
        int x = s.top();
        s.pop();
        if (!ms.empty() && x <= ms.top()) ms.pop();
    }
    
    int top() {
        return s.top();
    }
    
    int getMin() {
        return ms.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();
 */

挺巧妙的，之前一直没想出来怎么O(1)查最小值....