cs: add «1639. Number of Ways to Form a Target String Given a Dictionary»

URL:	https://leetcode.com/problems/number-of-ways-to-form-a-target-string-given-a-dictionary/
Signed-off-by: Matej Focko <me@mfocko.xyz>

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

@@ -0,0 +1,56 @@
+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;
+    }
+}