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Task/ABC-Problem/D/abc-problem-4.d

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+import std.stdio, std.ascii, std.algorithm, std.array, std.range;
+
+alias Block = char[2];
+
+bool canMakeWord(immutable Block[] blocks, in string word) pure nothrow
+in {
+    assert(blocks.all!(w => w[].all!isAlpha));
+    assert(word.all!isAlpha);
+} body {
+    bool inner(size_t[] indexes, in string w) pure nothrow {
+        if (w.empty)
+            return true;
+
+        immutable c = w[0].toUpper;
+        foreach (ref idx; indexes) {
+            if (blocks[idx][0].toUpper != c &&
+                blocks[idx][1].toUpper != c)
+                continue;
+            indexes[0].swap(idx);
+            if (inner(indexes[1 .. $], w[1 .. $]))
+                return true;
+            indexes[0].swap(idx);
+        }
+
+        return false;
+    }
+
+    return inner(blocks.length.iota.array, word);
+}
+
+void main() {
+    enum Block[] blocks = "BO XK DQ CP NA GT RE TG QD FS
+                           JW HU VI AN OB ER FS LY PC ZM".split;
+
+    foreach (w; "" ~ "A BARK BoOK TrEAT COmMoN SQUAD conFUsE".split)
+        writefln(`"%s" %s`, w, blocks.canMakeWord(w));
+
+    // Extra test.
+    immutable Block[] blocks2 = ["AB", "AB", "AC", "AC"];
+    immutable word = "abba";
+    writefln(`"%s" %s`, word, blocks2.canMakeWord(word));
+}