Leetcode1475. Final Prices With a Special Discount in a Shop (单调栈经典题)

1475. 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

解法1: 单调栈。注意res[]里面存储的是discount的值。

class Solution {
public:
    vector<int> finalPrices(vector<int>& prices) {
        int n = prices.size();
        vector<int> res = prices;
        stack<int> stk;
        for (int i = 0; i < n; i++) {
            while (!stk.empty() && prices[stk.top()] >= prices[i]) {
                res[stk.top()] = prices[stk.top()] - prices[i];
                stk.pop();
            }
            stk.push(i);
        }
        return res;
    }
};
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