903 unfinish

Author: aQuaYi

Algorithms/0903.valid-permutations-for-di-sequence/valid-permutations-for-di-sequence.go

@@ -1,6 +1,32 @@
 package problem0903
 
+const mod = 1e9 + 7
+
+// ref: https://leetcode.com/problems/valid-permutations-for-di-sequence/discuss/168278/C++JavaPython-DP-Solution-O(N2)
 func numPermsDISequence(S string) int {
+	n := len(S)
+
+	dp := [201][201]int{}
+
+	for j := 0; j <= n; j++ {
+		dp[0][j] = 1
+	}
+
+	for i := 0; i < n; i++ {
+		count := 0
+		if S[i] == 'I' {
+			for j := 0; j < n-i; j++ {
+				count += dp[i][j]
+				dp[i+1][j] = count % mod
+
+			}
+		} else {
+			for j := n - i - 1; j >= 0; j-- {
+				count += dp[i][j+1]
+				dp[i+1][j] = count % mod
+			}
+		}
+	}
 
-	return 0
+	return dp[n][0]
 }

Algorithms/0903.valid-permutations-for-di-sequence/valid-permutations-for-di-sequence_test.go

@@ -18,6 +18,11 @@ var tcs = []struct {
 		5,
 	},
 
+	{
+		"DIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDDDIDD",
+		795732224,
+	},
+
 	// 可以有多个 testcase
 }