先使用最直观的解法：

class CustomStack {
public:
    CustomStack(int maxSize) {
        m_maxsize = maxSize;
    }
    
    void push(int x) {
        if(m_elem.size() >= m_maxsize)
            return;
        m_elem.push(x);
    }
    
    int pop() {
        if(m_elem.empty())
            return -1;
        int x = m_elem.top();
        m_elem.pop();
        return x;
    }
    
    void increment(int k, int val) {
        stack<int> exchange;
        while(!m_elem.empty())
        {
            int elem = m_elem.top();
            m_elem.pop();
            exchange.push(elem);
        }
        
        while(!exchange.empty())
        {
            int elem = exchange.top();
            exchange.pop();
            if(k-- > 0)
                elem += val;
            m_elem.push(elem);
        }
    }
private:
    int m_maxsize;
    stack<int> m_elem;
};

/**
 * 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);
 */

运行效率：

Runtime: 96 ms, faster than 6.13% of C++ online submissions for Design a Stack With Increment Operation.
Memory Usage: 37.2 MB, less than 6.22% of C++ online submissions for Design a Stack With Increment Operation.

使用差分数组后，代码如下：

class CustomStack {
public:
    CustomStack(int maxSize) {
        m_size = maxSize;
    }
    
    void push(int x) {
        if(stack.size() == m_size) return;
        stack.push_back(x);
        inc.push_back(0);
    }
    
    int pop() {
        int i = stack.size() - 1;
        
        if(i < 0) return -1;
        if(i > 0) inc[i-1] += inc[i];
        
        int elem = stack[i] + inc[i];
       
        stack.pop_back();
        inc.pop_back();
        return elem;
    }
    
    void increment(int k, int val) {
        int i = min(k,(int)stack.size()) - 1;
        if(i >= 0)  inc[i] += val;
    }
private:
    vector<int> stack;
    vector<int> inc;
    int m_size;
};

/**
 * 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);
 */

运行效率：

Runtime: 36 ms, faster than 44.61% of C++ online submissions for Design a Stack With Increment Operation.
Memory Usage: 20.9 MB, less than 68.74% of C++ online submissions for Design a Stack With Increment Operation.