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

Solution

Fill in the candy array in two directions.

For each direction, we only consider the neighbor from (left/right) sides.

    public int candy(int[] ratings) {
        int n = ratings.length;
        int ret = 0;
        int[] candy = new int[n];
        for(int i = 0; i < n; i++){
            candy[i] = 1;
        }
        
        for(int i = 1; i < n; i++){
            if(ratings[i] > ratings[i-1]){
                candy[i] = candy[i-1]+1;
            }
        }
        
        for(int i = n-2; i >= 0; i--){
            if(ratings[i] > ratings[i+1]){
                candy[i] = Math.max(candy[i+1]+1, candy[i]);
            }
        }
        
        for(int num : candy){
            ret+=num;
        }
        return ret;
    }