leetcode 155

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.

设计支持push、pop、top和在常量时间内检索最小元素的栈。 
push(x) —— 推送元素X进栈 
pop() —— 移除栈顶元素
top() —— 得到栈顶元素
getMin() —— 检索栈的最小元素
利用两个vector来实现，一个实现正常的栈，一个在压栈的时候存储最小元素，在检索栈的最小元素的时候直接取最小栈的栈顶元素即可。
代码如下：

class MinStack {
public:
    /** initialize your data structure here. */
    vector<int> allstack;
    vector<int> minstack;
    MinStack() {
        
    }
    
    void push(int x) {
        if(allstack.empty())
        {
            allstack.push_back(x);
            minstack.push_back(x);
        }
        else
        {
            allstack.push_back(x);
            if(x <= minstack[minstack.size()-1])
            {
                minstack.push_back(x);
            }
        }
    }
    
    void pop() {
        if(allstack[allstack.size()-1] == minstack[minstack.size()-1])
        {
            minstack.pop_back();
        }
        allstack.pop_back();
    }
    
    int top() {
        return allstack[allstack.size()-1];
    }
    
    int getMin() {
        return minstack[minstack.size()-1];
    }
};

/**
 * 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();
 */