leetcode — candy

/**
 * Source : https://oj.leetcode.com/problems/candy/
 *
 * 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:
 *
 * Each child must have at least one candy.
 * Children with a higher rating get more candies than their neighbors.
 *
 * What is the minimum candies you must give?
 *
 */
public class Candy {

    /**
     * 给站成一排的孩子分糖果，
     * 每个孩子至少一个
     * 每个孩子的权重不一样，权重高的孩子比相邻的孩子要多
     * 求最少需要多少个糖果
     *
     * 从左到右遍历，寻找递增序列，该位置对应的孩子分的糖果数也递增，每个递增序列从1开始，这样保证了rating高的孩子比左边的孩子多
     * 然后从右向左遍历，确保当前位置孩子的糖果数多于右边孩子的
     * 复杂度为O(n)
     *
     * @param ratings
     * @return
     */
    public int candy (int[] ratings) {
        if (ratings.length <= 0) {
            return 0;
        }
        int[] candy = new int[ratings.length];
        candy[0] = 1;
        for (int i = 1; i < ratings.length; i++) {
            if (ratings[i] > ratings[i-1]) {
                candy[i] = candy[i-1] + 1;
            } else {
                candy[i] = 1;
            }
        }
        int sum = candy[ratings.length-1];
        for (int i = ratings.length-2; i > -1; i--) {
            if (ratings[i] > ratings[i+1] && candy[i] <= candy[i+1]) {
                candy[i] = candy[i+1] + 1;
            }
            sum += candy[i];
        }
        return sum;
    }

    public static void main(String[] args) {
        Candy candy = new Candy();
        int[] ratings = new int[]{5, 6, 7, 4, 1, 2, 3, 2, 1, 7};
        System.out.println(candy.candy(ratings) + " == 19");
    }
}