Create 2179-count-good-triplets-in-an-array.js

Author: everthis

2179-count-good-triplets-in-an-array.js

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+/**
+ * @param {number[]} a
+ * @param {number[]} b
+ * @return {number}
+ */
+const goodTriplets = function(a, b) {
+    let n = a.length, m = new Map(), res = 0;
+    for (let i = 0; i < n; i++) m.set(b[i], i);
+    let fen = new Fenwick(n + 3);
+    for (let i = 0; i < n; i++) {
+       let pos = m.get(a[i]);
+       let l = fen.query(pos), r = (n - 1 - pos) - (fen.query(n - 1) - fen.query(pos));
+       res += l * r; 
+       fen.update(pos, 1);
+    }
+    return res;
+};
+function Fenwick(n) {
+    let tree = Array(n).fill(0);
+    return { query, update }
+    function query(i) {
+        let sum = 0;
+        i++;
+        while (i > 0) {
+            sum += tree[i];
+            i -= i & -i;
+        }
+        return sum;
+    }
+    function update(i, v) {
+        i++;
+        while (i < n) {
+            tree[i] += v;
+            i += i & -i;
+        }
+    }
+}