Create valid-permutations-for-di-sequence.cpp

rui-yan · Sep 9, 2018 · 98fba40 · 98fba40
1 parent b779373
commit 98fba40
Showing 1 changed file with 24 additions and 0 deletions.
diff --git a/C++/valid-permutations-for-di-sequence.cpp b/C++/valid-permutations-for-di-sequence.cpp
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+// Time:  O(n^2)
+// Space: O(n)
+
+class Solution {
+public:
+    int numPermsDISequence(string S) {
+        const int M = 1e9 + 7;
+        vector<vector<int>> dp(2, vector<int>(S.length() + 1, 1));
+        for (int i = 0; i < S.length(); ++i) {
+            if (S[i] == 'I') {
+                dp[(i + 1) % 2][0] = dp[i % 2][0];
+                for (int j = 1; j < S.length() - i; ++j) {
+                    dp[(i + 1) % 2][j] = (dp[(i + 1) % 2][j - 1] + dp[i % 2][j]) % M;
+                }
+            } else {
+                dp[(i + 1) % 2][S.length() - i - 1] = dp[i % 2][S.length() - i];
+                for (int j = S.length() - i - 2; j >= 0; --j) {
+                    dp[(i + 1) % 2][j] = (dp[(i + 1) % 2][j + 1] + dp[i % 2][j + 1]) % M;
+                }
+            }
+        }
+        return dp[S.length() % 2][0];
+    }
+};