1894

O(n) solution
class Solution {
public:
    int chalkReplacer(vector<int>& chalk, int k) {
        k %= accumulate(begin(chalk), end(chalk), 0l);
        for(int i = 0; i < chalk.size(); ++i)
            if((k -= chalk[i]) < 0)
                return i;
        return 0;
    }
};

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O(n) solution

class Solution {
public:
    int chalkReplacer(vector<int>& c, int k) {
        long long n = c.size();
        vector<long long> chalk(n);
        chalk[0] = c[0];
        for(int i=1; i<chalk.size(); i++)
            chalk[i] = chalk[i-1]+c[i];
        k = k%chalk[n-1];
        if(k == 0) return 0;
        int pos = upper_bound(chalk.begin(), chalk.end(), k)-chalk.begin();
        return pos;
    }
};