Create 1395-count-number-of-teams.js

Author: everthis

1395-count-number-of-teams.js

@@ -0,0 +1,64 @@
+/**
+ * @param {number[]} rating
+ * @return {number}
+ */
+const numTeams = function(rating) {
+  let res = 0
+  for(let i = 1, n = rating.length; i < n - 1; i++) {
+    const less = Array(2).fill(0), greater = Array(2).fill(0)
+    for(let j = 0; j < n; j++) {
+      if(rating[i] > rating[j]) {
+        less[j < i ? 0 : 1]++
+      }
+      if(rating[i] < rating[j]) {
+        greater[j > i ? 0 : 1]++
+      }
+    }
+    res += less[0] * greater[0] + less[1] * greater[1]
+  }
+  return res
+};
+
+
+// another
+
+/**
+ * @param {number[]} rating
+ * @return {number}
+ */
+const numTeams = function(rating) {
+  if(rating.length < 3) return 0
+  const n = rating.length
+  const leftTree = Array(1e5 + 1).fill(0)
+  const rightTree = Array(1e5 + 1).fill(0)
+  for(let r of rating) update(rightTree, r, 1)
+  let res = 0
+  for(let r of rating) {
+    update(rightTree, r,  -1)
+    res += getPrefixSum(leftTree, r - 1) * getSuffixSum(rightTree, r + 1)
+    res += getSuffixSum(leftTree, r + 1) * getPrefixSum(rightTree, r - 1)
+    update(leftTree, r, 1)
+  }
+
+  return res
+};
+
+function update(bit, index, val) {
+  while(index < bit.length) {
+    bit[index] += val
+    index += index & (-index)
+  }
+}
+
+function getPrefixSum(bit, index) {
+  let res = 0
+  while(index > 0) {
+    res += bit[index]
+    index -= index & (-index)
+  }
+  return res
+}
+
+function  getSuffixSum(bit, index) {
+  return getPrefixSum(bit, 1e5) - getPrefixSum(bit, index - 1)
+}