Leetcode 135. Candy

There are n children standing in a line. Each child is assigned a rating value given in the integer array ratings.

You are giving candies to these children subjected to the following requirements:

• Each child must have at least one candy.
• Children with a higher rating get more candies than their neighbors.

Return the minimum number of candies you need to have to distribute the candies to the children.

Example 1:Input: ratings = [1,0,2] Output: 5 Explanation: You can allocate to the first, second and third child with 2, 1, 2 candies respectively.

Example 2:Input: ratings = [1,2,2] Output: 4 Explanation: You can allocate to the first, second and third child with 1, 2, 1 candies respectively. The third child gets 1 candy because it satisfies the above two conditions.

Constraints:

• n == ratings.length
• 1 <= n <= 2 * 104
• 0 <= ratings[i] <= 2 * 104

Based on the problem description, it seems a bilt complicated, but can be simply soved by greedy. However, it might be hard to realize how to apply greedy to this problem.

class Solution:
    def candy(self, ratings: List[int]) -> int:
        candies = [1] * len(ratings)

        for i in range(1, len(ratings)):
            if ratings[i] > ratings[i-1]:
                candies[i] = candies[i-1] + 1
        
        for i in range(len(ratings)-2, -1, -1):
            if ratings[i] > ratings[i+1] and candies[i] <= candies[i+1]:
                candies[i] = candies[i+1] + 1
        
        return sum(candies)