1475. Final Prices With a Special Discount in a Shop

leetcode_1475.java

/**
1475. Final Prices With a Special Discount in a Shop
https://leetcode.com/problems/final-prices-with-a-special-discount-in-a-shop
Easy

You are given an integer array prices where prices[i] is the price of the ith item in a shop.

There is a special discount for items in the shop. If you buy the ith item, then you will receive a discount equivalent to prices[j] where j is the minimum index such that j > i and prices[j] <= prices[i]. Otherwise, you will not receive any discount at all.

Return an integer array answer where answer[i] is the final price you will pay for the ith item of the shop, considering the special discount.

Example 1:

Input: prices = [8,4,6,2,3]
Output: [4,2,4,2,3]
Explanation: 
For item 0 with price[0]=8 you will receive a discount equivalent to prices[1]=4, therefore, the final price you will pay is 8 - 4 = 4.
For item 1 with price[1]=4 you will receive a discount equivalent to prices[3]=2, therefore, the final price you will pay is 4 - 2 = 2.
For item 2 with price[2]=6 you will receive a discount equivalent to prices[3]=2, therefore, the final price you will pay is 6 - 2 = 4.
For items 3 and 4 you will not receive any discount at all.

Example 2:

Input: prices = [1,2,3,4,5]
Output: [1,2,3,4,5]
Explanation: In this case, for all items, you will not receive any discount at all.
Example 3:

Input: prices = [10,1,1,6]
Output: [9,0,1,6]
 

Constraints:

1 <= prices.length <= 500
1 <= prices[i] <= 1000

*/

class Solution {
    public int[] finalPrices(int[] prices) {
        int n = prices.length;
        int[] stack = new int[n];f
        int head = -1;
        int[] res = new int[n];
        for (int i = 0; i < n; i++) {
            while (head != -1 && prices[stack[head]] >= prices[i]) {
                int idx = stack[head--];
                res[idx] = prices[idx] - prices[i];
            }
            stack[++head] = i;
        }
        while (head != -1) {
            int idx = stack[head--];
            res[idx] = prices[idx];
        }
        return res;
    }
}