candy.md

title	description	authors	tags	categories	date	lastmod	draft	archive
Candy	Some description ...	lek-tin	leetcode greedy	algorithm	2020-03-09 00:42:43 -0700	2020-03-09 00:42:43 -0700	false	false

There are N children standing in a line. Each child is assigned a rating value.

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

1. Each child must have at least one candy.
2. Children with a higher rating get more candies than their neighbors.
3. What is the minimum candies you must give?

Example 1

Input: [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: [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.

Solution (greedy bi-directional scans)

Time: O(N)
Space: O(N)

class Solution:
    def candy(self, ratings: List[int]) -> int:
        if not ratings or len(ratings)==0:
            return 0

        total = 0
        leftToRight = [1] * len(ratings)
        rightToLeft = [1] * len(ratings)

        for i in range(1, len(ratings)):
            if ratings[i] > ratings[i-1]:
                leftToRight[i] = leftToRight[i-1]+1
        for i in range(len(ratings)-2, -1, -1):
            if ratings[i] > ratings[i+1]:
                rightToLeft[i] = rightToLeft[i+1]+1
        for i in range(len(ratings)):
            total += max(leftToRight[i], rightToLeft[i])

        return total