20250415 finished.

src/problem/mod.rs

@@ -599,3 +599,5 @@ mod p3272_find_the_count_of_good_integers;
 mod p1922_count_good_numbers;
 
 mod p1534_count_good_triplets;
+
+mod p2179_count_good_triplets_in_an_array;

src/problem/p2179_count_good_triplets_in_an_array.rs

@@ -0,0 +1,99 @@
+/**
+ * [2179] Count Good Triplets in an Array
+ */
+pub struct Solution {}
+
+// submission codes start here
+
+struct FenwickTree {
+    tree: Vec<i64>,
+}
+
+impl FenwickTree {
+    fn new(size: usize) -> Self {
+        Self {
+            tree: vec![0; size + 1],
+        }
+    }
+
+    fn update(&mut self, mut index: usize, delta: i64) {
+        index += 1;
+
+        while index < self.tree.len() {
+            self.tree[index] += delta;
+
+            index += Self::low_bit(index)
+        }
+    }
+
+    fn query(&self, mut index: usize) -> i64 {
+        index += 1;
+        let mut result = 0;
+
+        while index > 0 {
+            result += self.tree[index];
+            index -= Self::low_bit(index)
+        }
+
+        result
+    }
+
+    // 二进制最低位1以及后面的0组成的数
+    fn low_bit(i: usize) -> usize {
+        i & (!i + 1)
+    }
+}
+
+impl Solution {
+    pub fn good_triplets(nums1: Vec<i32>, nums2: Vec<i32>) -> i64 {
+        let n = nums1.len();
+        // nums2中各个值到下标的映射数组
+        let mut pos2 = vec![0; n];
+        // index_mapping 是nums1中一个数在nums2中的下标
+        // reverse_index_mapping是该下标同nums1中数字下标的对应关系
+        let mut reversed_index_mapping = vec![0; n];
+
+        for (i, v) in nums2.iter().enumerate() {
+            pos2[*v as usize] = i;
+        }
+
+        for i in 0..n {
+            // nums1中的一个值在nums2中的下标到该值在nums1中的下标的映射
+            reversed_index_mapping[pos2[nums1[i] as usize]] = i;
+        }
+
+        let mut tree = FenwickTree::new(n);
+
+        let mut result = 0;
+
+        // 枚举num1s中的一个值在nums2中的下标
+        for j in 0..n {
+            let pos = reversed_index_mapping[j];
+            let left = tree.query(pos);
+            tree.update(pos, 1);
+            let right = ((n - 1 - pos) - (j - left as usize)) as i64;
+            result += left * right;
+        }
+
+        result
+    }
+}
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_2179() {
+        assert_eq!(
+            1,
+            Solution::good_triplets(vec![2, 0, 1, 3], vec![0, 1, 2, 3])
+        );
+        assert_eq!(
+            4,
+            Solution::good_triplets(vec![4, 0, 1, 3, 2], vec![4, 1, 0, 2, 3])
+        );
+    }
+}