135-candy

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Description

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

Solution(Python)

class Solution:
    def candy(self, ratings: List[int]) -> int:
        return self.SinglePassConstantSpace(ratings)

    # Time Complexity: O(n^2)
    # Space Complexity: O(n)
    def bruteforce(self, ratings: List[int]) -> int:
        n = len(ratings)
        candies = [1] * n
        hasChanged = True

        while hasChanged:
            hasChanged = False

            for i in range(n):
                if (
                    i != n - 1
                    and ratings[i] > ratings[i + 1]
                    and candies[i] <= candies[i + 1]
                ):
                    candies[i] = candies[i + 1] + 1
                    hasChanged = True
                if (
                    i >= 0
                    and ratings[i] > ratings[i - 1]
                    and candies[i] <= candies[i - 1]
                ):
                    candies[i] = candies[i - 1] + 1
                    hashChanged = True
        return sum(candies)

    # Time Complexity:O(n)
    # Space Complexity: O(n)
    def presum(self, ratings: List[int]) -> int:
        n = len(ratings)
        candies = [1] * n

        for i in range(1, n):
            if ratings[i] > ratings[i - 1]:
                candies[i] = candies[i - 1] + 1

        for i in range(n - 2, -1, -1):
            if ratings[i] > ratings[i + 1]:
                candies[i] = max(candies[i + 1] + 1, candies[i])

        return sum(candies)

    # Time Complexity:O(n)
    # Space Complexity: O(1)
    def SinglePassConstantSpace(self, ratings: List[int]) -> int:
        n = len(ratings)
        if n <= 1:
            return n

        def summateN(n):
            return (n * (n + 1)) // 2

        candies = 0
        up = 0
        down = 0
        oldSlope = 0

        for i in range(1, n):
            newSlope = (
                1
                if ratings[i] > ratings[i - 1]
                else -1
                if ratings[i] < ratings[i - 1]
                else 0
            )

            if oldSlope > 0 and newSlope == 0 or (oldSlope < 0 and newSlope >= 0):
                candies += summateN(up) + summateN(down) + max(up, down)
                up = 0
                down = 0

            if newSlope > 0:
                up += 1
            elif newSlope < 0:
                down += 1
            else:
                candies += 1
            oldSlope = newSlope

        candies += summateN(up) + summateN(down) + max(up, down) + 1
        return candies