LeetCode/cs/number-of-ways-to-form-a-target-string-given-a-dictionary.cs

public class Solution {
    private static readonly int MOD = 1000000007;

    private static int[,] GetFreqs(string[] words) {
        var freqs = new int[words[0].Length, 26];

        foreach (var word in words) {
            for (int i = 0; i < word.Length; ++i) {
                ++freqs[i, word[i] - 'a'];
            }
        }

        return freqs;
    }

    private struct Counter {
        public long Previous;
        public long Current;
    }

    private static void PushCurrent(Counter[] counters) {
        for (var i = 0; i < counters.Length; ++i) {
            counters[i].Current = counters[i].Previous;
        }
    }

    private static void PopCurrent(Counter[] counters) {
        for (var i = 0; i < counters.Length; ++i) {
            counters[i].Previous = counters[i].Current;
        }
    }

    public int NumWays(string[] words, string target) {
        var freqs = GetFreqs(words);

        var counters = new Counter[target.Length + 1];
        counters[0].Previous = 1;

        for (var length = 1; length <= words[0].Length; ++length) {
            PushCurrent(counters);

            for (var j = 1; j <= target.Length; ++j) {
                var pos = target[j - 1] - 'a';

                counters[j].Current +=
                    freqs[length - 1, pos] * counters[j - 1].Previous
                 % MOD;
                counters[j].Current %= MOD;
            }

            PopCurrent(counters);
        }

        return (int)counters[target.Length].Previous;
    }
}