135. Candy

Problem Statement

Hard

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

Java — 2 pass

Source file

class Solution {
    public int candy(int[] ratings) {
        int n = ratings.length, ans = 0;
        int[] candies = new int[n];
        Arrays.fill(candies, 1);
        for (int i = 1; i < n; i++)
            if (ratings[i - 1] < ratings[i])
                candies[i] = candies[i - 1] + 1;
        for (int i = n - 1; i > 0; i--) {
            if (ratings[i - 1] > ratings[i])
                candies[i - 1] = Math.max(candies[i - 1], candies[i] + 1);
            ans += candies[i - 1];
        }
        return ans += candies[n - 1];
    }
}

Java

Source file

class Solution {
    public int candy(int[] ratings) {
        int n = ratings.length, ans = 0;
        int[] candies = new int[n];
        Arrays.fill(candies, 1);
        for (int i = 1; i < n; i++)
            if (ratings[i - 1] < ratings[i])
                candies[i] = candies[i - 1] + 1;
        for (int i = n - 1; i > 0; i--) {
            if (ratings[i - 1] > ratings[i])
                candies[i - 1] = Math.max(candies[i - 1], candies[i] + 1);
            ans += candies[i - 1];
        }
        return ans += candies[n - 1];
    }
}