27 lines
786 B
Rust
27 lines
786 B
Rust
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impl Solution {
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pub fn k_inverse_pairs(n: i32, k: i32) -> i32 {
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let MOD = 1000000007;
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let idx = |i, j| (i * (k + 1) + j) as usize;
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let get = |dp: &[i32], i: i32, j: i32| {
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if i < 0 || i > n || j < 0 || j > k {
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return 0;
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}
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dp[idx(i, j)]
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};
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let mut dp = vec![0; ((n + 1) * (k + 1)) as usize];
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for (i, j) in (1..=n).flat_map(|i| (0..=k).map(move |j| (i, j))) {
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if j == 0 {
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dp[idx(i, j)] = 1;
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} else {
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let val = (get(&dp, i - 1, j) + MOD - get(&dp, i - 1, j - i)) % MOD;
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dp[idx(i, j)] = (get(&dp, i, j - 1) + val) % MOD;
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}
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}
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(dp[idx(n, k)] + MOD - get(&dp, n, k - 1)) % MOD
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}
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}
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