Create 3333. Find the Original Typed String II

3333. Find the Original Typed String II

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+class Solution {
+    static constexpr int MOD = 1'000'000'007;
+public:
+    int possibleStringCount(string word, int k) {
+        vector<int> groups;
+        for(int i = 0, n = word.size(); i < n; ) {
+            int j = i+1;
+            while(j<n && word[j]==word[i]) j++;
+            groups.push_back(j - i);
+            i = j;
+        }
+
+        long long total = 1;
+        for(int g : groups)
+            total = total * g % MOD;
+
+        int m = groups.size();
+        if(k > (int)groups.size()) {
+            vector<int> dp(k), newdp(k);
+            dp[0] = 1;
+            for(int idx = 0; idx < m; idx++) {
+                int len = groups[idx];
+                fill(newdp.begin(), newdp.end(), 0);
+
+                long long window = 0;
+                for(int j = 1; j < k; j++) {
+                    window = (window + dp[j-1]) % MOD;
+                    if(j - 1 - len >= 0)
+                        window = (window - dp[j - 1 - len] + MOD) % MOD;
+                    newdp[j] = window;
+                }
+                dp.swap(newdp);
+            }
+
+            long long invalid = 0;
+            for(int j = 1; j < k; j++)
+                invalid = (invalid + dp[j]) % MOD;
+
+            return int((total - invalid + MOD) % MOD);
+        }
+        // If minimum intended length <= #groups, we don't exclude anything
+        return int(total);
+    }
+};