Formatted question description: https://leetcode.ca/all/1381.html

# 1381. Design a Stack With Increment Operation (Medium)

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.

Example 1:

Input
["CustomStack","push","push","pop","push","push","push","increment","increment","pop","pop","pop","pop"]
[,,,[],,,,[5,100],[2,100],[],[],[],[]]
Output
[null,null,null,2,null,null,null,null,null,103,202,201,-1]
Explanation
CustomStack customStack = new CustomStack(3); // Stack is Empty []
customStack.push(1);                          // stack becomes 
customStack.push(2);                          // stack becomes [1, 2]
customStack.pop();                            // return 2 --> Return top of the stack 2, stack becomes 
customStack.push(2);                          // stack becomes [1, 2]
customStack.push(3);                          // stack becomes [1, 2, 3]
customStack.push(4);                          // stack still [1, 2, 3], Don't add another elements as size is 4
customStack.increment(5, 100);                // stack becomes [101, 102, 103]
customStack.increment(2, 100);                // stack becomes [201, 202, 103]
customStack.pop();                            // return 103 --> Return top of the stack 103, stack becomes [201, 202]
customStack.pop();                            // return 202 --> Return top of the stack 102, stack becomes 
customStack.pop();                            // return 201 --> Return top of the stack 101, stack becomes []
customStack.pop();                            // return -1 --> Stack is empty return -1.


Constraints:

• 1 <= maxSize <= 1000
• 1 <= x <= 1000
• 1 <= k <= 1000
• 0 <= val <= 100
• At most 1000 calls will be made to each method of increment, push and pop each separately.

Related Topics:
Stack, Design

## Solution 1. Brute Force

// Time:
//      CustomStack, push, pop: O(1)
//      increment: O(k)
// Space: O(N)
class CustomStack {
int N;
vector<int> v;
public:
CustomStack(int maxSize): N(maxSize) {}
void push(int x) {
if (v.size() >= N) return;
v.push_back(x);
}
int pop() {
if (v.empty()) return -1;
int n = v.back();
v.pop_back();
return n;
}
void increment(int k, int val) {
k = min(k, (int)v.size());
for (int i = 0; i < k; ++i) v[i] += val;
}
};


## Solution 1.1. Queue

Use a double-ended queue deque. Also maintain a capacity and a size.

For the constructor, initialize deque, initialize capacity = maxSize and initialize size = 0.

For method push, if size < capacity, then add x to the end of deque and increase size by 1.

For method pop, if size > 0, then decrease size by 1, poll the first element of deque and return. Otherwise, return -1.

For method increment, set k to the minimum of k and size. For the k elements at the head of deque, poll them out, add val to each of them, and offer them back at the head of deque. Use a stack to temporally store the elements.

class CustomStack {
Deque<Integer> deque;
int capacity;
int size;

public CustomStack(int maxSize) {
capacity = maxSize;
size = 0;
}

public void push(int x) {
if (size < capacity) {
deque.offerLast(x);
size++;
}
}

public int pop() {
if (size > 0) {
size--;
return deque.pollLast();
} else
return -1;
}

public void increment(int k, int val) {
k = Math.min(k, size);
Stack<Integer> tempStack = new Stack<Integer>();
for (int i = 0; i < k; i++) {
int num = deque.pollFirst() + val;
tempStack.push(num);
}
for (int i = 0; i < k; i++)
deque.offerFirst(tempStack.pop());
}
}


## Solution 2. Lazy Propogation

// Time:
//      CustomStack: O(N)
//      push, pop, increment: O(1)
// Space: O(N)
// Ref: https://leetcode.com/problems/design-a-stack-with-increment-operation/discuss/539716/JavaC%2B%2BPython-Lazy-increment-O(1)
class CustomStack {
stack<int> s;
vector<int> update;
public:
CustomStack(int maxSize) : update(maxSize) {}

void push(int x) {
if (s.size() == update.size()) return;
s.push(x);
}

int pop() {
if (s.empty()) return -1;
int ans = s.top(), index = s.size() - 1;
s.pop();
ans += update[index];
if (index - 1 >= 0) update[index - 1] += update[index];
update[index] = 0;
return ans;
}

void increment(int k, int val) {
k = min(k, (int)s.size()) - 1;
if (k >= 0) update[k] += val;
}
};