rs: add «629. K Inverse Pairs Array»

Signed-off-by: Matej Focko <me@mfocko.xyz>

rs/k-inverse-pairs-array.rs

@@ -0,0 +1,26 @@
+impl Solution {
+    pub fn k_inverse_pairs(n: i32, k: i32) -> i32 {
+        let MOD = 1000000007;
+
+        let idx = |i, j| (i * (k + 1) + j) as usize;
+        let get = |dp: &[i32], i: i32, j: i32| {
+            if i < 0 || i > n || j < 0 || j > k {
+                return 0;
+            }
+
+            dp[idx(i, j)]
+        };
+
+        let mut dp = vec![0; ((n + 1) * (k + 1)) as usize];
+        for (i, j) in (1..=n).flat_map(|i| (0..=k).map(move |j| (i, j))) {
+            if j == 0 {
+                dp[idx(i, j)] = 1;
+            } else {
+                let val = (get(&dp, i - 1, j) + MOD - get(&dp, i - 1, j - i)) % MOD;
+                dp[idx(i, j)] = (get(&dp, i, j - 1) + val) % MOD;
+            }
+        }
+
+        (dp[idx(n, k)] + MOD - get(&dp, n, k - 1)) % MOD
+    }
+}