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1475 - Final Prices With a Special Discount in a Shop (Easy)

https://leetcode.com/problems/final-prices-with-a-special-discount-in-a-shop/

Problem Statement​

Given the 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 array where the ith element 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] <= 10^3

Approach 1: Stack​

We first initialize the result list to be a copy of prices. The reason of choosing to copy prices is that it is convenient to not change anything if an item does not have a discount.

Then we initalize a stack and iterate prices. If the last element of the stack if greater than or equal to the current_price , we have found a discount for the last element of the stack. We calculate the discounted price, and put it in the result array (its index is stored as the second element). We pop that element from the stack.

We then add the new item to the stack with its value and its index.

Written by @heiheihang
def finalPrices(self, prices: List[int]) -> List[int]:
        
        #initialize the result list to be a copy of prices
        result = prices.copy()
        
        #initialize stack
        stack = []
        
        #iterate prices
        for i in range(len(prices)):
            
            #declare current_price
            current_price = prices[i]
            
            #identify the prices that have not found a discount yet
            while(stack and stack[-1][0] >= current_price):
                
                #get the item's index
                item_index = stack[-1][1]
                
                #set its discounted price
                result[item_index] = stack[-1][0] - current_price
                
                #remove the item as it has found a discount
                stack.pop()
            
            #add the current item to the stack
            stack.append([current_price, i])
        
        #return result
        return result