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@@ -0,0 +1,35 @@
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+const MOD int = 1000000007
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+
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+func kInversePairs(n int, k int) int {
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+ if k < 0 || n*(n-1)/2 < k { // For n, n-1, ... , 2, 1, k = n*(n-1)/2 -> maximum
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+ return 0
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+ } else if k == 0 || k == n*(n-1)/2 {
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+ return 1
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+ }
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+ dp := make([][]int, n+1)
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+ for i := range dp {
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+ dp[i] = make([]int, k+1)
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+ }
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+ // dp[n][k] means kIP(n, k), so:
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+ // dp[n][k] = dp[n-1][k] + dp[n-1][k-1] + ... + dp[n-1][k-n+1]
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+ // dp[n][k+1] = dp[n-1][k+1] + dp[n-1][k] + ... + dp[n-1][k+1-n+1]
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+ // => dp[n][k+1] = dp[n][k] + dp[n-1][k+1] - dp[n-1][k-n+1]
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+ dp[2][0], dp[2][1] = 1, 1
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+ for i := 3; i <= n; i++ {
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+ dp[i][0] = 1
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+ for j := 1; j <= minInt(k, i*(i-1)/2); j++ {
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+ dp[i][j] = (dp[i][j-1] + dp[i-1][j]) % MOD
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+ if j >= i {
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+ dp[i][j] = (dp[i][j] + MOD - dp[i-1][j-i]) % MOD
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+ }
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+ }
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+ }
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+ return dp[n][k]
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+}
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+
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+func minInt(x, y int) int {
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+ if x < y {
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+ return x
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+ }
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+ return y
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+}
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