feat: 2552. Count Increasing Quadruplets, dp

Author: upupming

leetcode/残酷刷题/2552. Count Increasing Quadruplets.js

@@ -0,0 +1,37 @@
+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+var countQuadruplets = function(nums) {
+  const n = nums.length
+  let ans = 0
+
+  const prev = Array(n+1).fill(0).map(() => Array(n+1).fill(0))
+  const post = Array(n+1).fill(0).map(() => Array(n+1).fill(0))
+  for (let i = 0; i < n; i++) {
+      // 数的范围只能是 1~n
+      for (let j = 1; j <= n; j++) {
+          if (nums[i] < j) prev[i][j] = (i<=0 ? 0 : prev[i-1][j])+1
+          else prev[i][j] = i<=0 ? 0 : prev[i-1][j]
+      }
+      let k = n-1-i
+      for (let j = 1; j <= n; j++) {
+          if (nums[k] > j) post[k][j] = post[k+1][j]+1
+          else post[k][j] = post[k+1][j]
+          // console.log(k, j, post[k][j])
+      }
+  }
+
+  for (let j = 0; j < n; j++) {
+      for (let k = j+1; k < n; k++) {
+          if (nums[k] < nums[j]) {
+              let l = j-1<0 ? 0 : prev[j-1][nums[k]]
+              let r = post[k+1][nums[j]]
+              // post[3][5]
+              // console.log(j, k, nums[j], nums[k], l, r)
+              ans += l*r
+          }
+      }
+  }
+  return ans
+};