lc155-Min Stack

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class MinStack {
public:
    vector<int> a;
    int min;
    int len;
    bool flag;
    int min_index;
    /** initialize your data structure here. */
    MinStack() {
        len = 0;
        min = -1;
        flag = false;
    }
    
    void push(int x) {
        a.push_back(x);
        len++;
        if(!flag) {
            min = x;
            flag = true;
        }
        else {
            if (x < min) {
                min = x;
               min_index = len-1;
        }
        }
        
    }
    
    void pop() {
        a.pop_back();
        if(min_index == len - 1)
            get_min();
        len--;
        if(len == 0)
            flag = false;
    }
    
    int top() {
        return a[a.size()-1];
    }
    
    int getMin() {
        if(flag == false) {
            get_min();
        }
        return min;
    }
    
    void get_min() {
        min = a[0];
            for(int i = 0; i < a.size(); i++) {
                if(a[i] < min) {
                    min = a[i];
                    min_index = i;
                }
            }
    }
};
/**
 * 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();
 */