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?

Solution: You may refer to https://github.com/AnnieKim/ITint5/blob/master/031_%E5%88%86%E9%85%8D%E7%B3%96%E6%9E%9C.cpp
1. traverse only once with O(1) space. 2. O(n) space.

class Solution {
public:
    int candy(vector<int> &ratings) {
        int N = ratings.size();
        vector<int> candy(N,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] <= candy[i+1]) {
                candy[i] = candy[i+1] + 1;
            }
        }
        int res = 0;
        for(int i = 0; i < N; i++) {
            res += candy[i];
        }
        return res;
    }
};