Untitled

// leetcode 394 ===================================================

class Solution {
    public String decodeString(String s) {
        Stack<Character> st = new Stack<>();

        for(int i=0; i<s.length(); i++){
            char ch = s.charAt(i);

            if(ch != ']'){
                st.push(ch);
            } else {
                // retreive string after '['
                StringBuilder sb = new StringBuilder();

                while(st.size()>0 && st.peek()!='['){
                    sb.insert(0,st.pop());
                }

                String toRepeat = sb.toString();
                st.pop(); // removing '['

                // retreive digits before '['
                sb = new StringBuilder();
                while(st.size()>0 && st.peek()>='0' && st.peek()<='9'){
                    sb.insert(0,st.pop());
                }

                int count = Integer.parseInt(sb.toString());

                // push string "repeating" times
                while(count > 0){
                    for(char c: toRepeat.toCharArray()){
                        st.push(c);
                    }

                    count--;
                }
            }
        }

        StringBuilder ans = new StringBuilder();

        while(st.size()>0){
            ans.append(st.pop());
        }

        ans.reverse();
        return ans.toString();
    }
}



// next larger on left 

public static int[] nextLargerLeft(int[] arr){
        int n = arr.length;

        int[] ans = new int[n];

        Stack<Integer> st = new Stack<>();
        st.push(-1);

        for(int i=0; i<arr.length; i++){

            while(st.peek()!=-1 && arr[st.peek()] < arr[i]){
                st.pop();
            }

            int daysBefore = i - st.top();
            ans[i] = daysBefore;

            st.push(i);
        }

        return ans;
    }
    
// leetcode 901 ==============================================
class StockSpanner {
    Stack<int[]> st = new Stack<>();
    int currentDay = 0;
    public StockSpanner() {
        st.push(new int[]{-1,-1});
    }
    
    public int next(int price) {
        while(st.peek()[0]!=-1 && st.peek()[1] <= price){
            st.pop();
        }

        int daysBefore = currentDay - st.peek()[0];

        st.push(new int[]{currentDay,price});
        currentDay++;

        return daysBefore;
    }
}
 
 // leetcode 901 with class =========================================
 
 class StockSpanner {
    class DayPrice {
        int currentDay;
        int currentPrice;

        public DayPrice(int currentDay, int currentPrice){
            this.currentDay = currentDay;
            this.currentPrice = currentPrice;
        }
    }

    Stack<DayPrice> st = new Stack<>();
    int currentDay = 0;
    public StockSpanner() {
        st.push(new DayPrice(-1,-1));
    }
    
    public int next(int price) {
        while(st.peek().currentDay!=-1 && st.peek().currentPrice <= price){
            st.pop();
        }

        int daysBefore = currentDay - st.peek().currentDay;

        st.push(new DayPrice(currentDay, price));
        currentDay++;

        return daysBefore;
    }
}