1381. Design a Stack With Increment Operation

Design a stack which supports the following operations.

Implement the CustomStack class:

• CustomStack(int maxSize) Initializes the object with maxSize which is the maximum number of elements in the stack or do nothing if the stack reached the maxSize.
• void push(int x) Adds x to the top of the stack if the stack hasn’t reached the maxSize.
• int pop() Pops and returns the top of stack or -1 if the stack is empty.
• void inc(int k, int val) Increments the bottom k elements of the stack by val. If there are less than k elements in the stack, just increment all the elements in the stack.

class CustomStack {
    vector<pair<int, int>> st;
    int msz;
public:
    CustomStack(int maxSize): msz(maxSize) {
        st.reserve(msz);
    }

    void push(int x) {
        if(st.size() < msz) st.push_back({x, 0});
    }

    int pop() {
        if(st.empty()) return -1;
        int res = st.back().first + st.back().second;
        if(st.size() != 1) {
            st[st.size() - 2].second += st.back().second;
        }
        st.pop_back();

        return res;
    }

    void increment(int k, int val) {
        if(!st.empty()) {
            st[min(k, (int) st.size()) - 1].second += val;
        }
    }
};

/**
 * Your CustomStack object will be instantiated and called as such:
 * CustomStack* obj = new CustomStack(maxSize);
 * obj->push(x);
 * int param_2 = obj->pop();
 * obj->increment(k,val);
 */