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| 1 | +/** |
| 2 | + * @param {number[]} nums |
| 3 | + * @return {number} |
| 4 | + */ |
| 5 | +var countQuadruplets = function(nums) { |
| 6 | + const n = nums.length |
| 7 | + let ans = 0 |
| 8 | + |
| 9 | + const prev = Array(n+1).fill(0).map(() => Array(n+1).fill(0)) |
| 10 | + const post = Array(n+1).fill(0).map(() => Array(n+1).fill(0)) |
| 11 | + for (let i = 0; i < n; i++) { |
| 12 | + // 数的范围只能是 1~n |
| 13 | + for (let j = 1; j <= n; j++) { |
| 14 | + if (nums[i] < j) prev[i][j] = (i<=0 ? 0 : prev[i-1][j])+1 |
| 15 | + else prev[i][j] = i<=0 ? 0 : prev[i-1][j] |
| 16 | + } |
| 17 | + let k = n-1-i |
| 18 | + for (let j = 1; j <= n; j++) { |
| 19 | + if (nums[k] > j) post[k][j] = post[k+1][j]+1 |
| 20 | + else post[k][j] = post[k+1][j] |
| 21 | + // console.log(k, j, post[k][j]) |
| 22 | + } |
| 23 | + } |
| 24 | + |
| 25 | + for (let j = 0; j < n; j++) { |
| 26 | + for (let k = j+1; k < n; k++) { |
| 27 | + if (nums[k] < nums[j]) { |
| 28 | + let l = j-1<0 ? 0 : prev[j-1][nums[k]] |
| 29 | + let r = post[k+1][nums[j]] |
| 30 | + // post[3][5] |
| 31 | + // console.log(j, k, nums[j], nums[k], l, r) |
| 32 | + ans += l*r |
| 33 | + } |
| 34 | + } |
| 35 | + } |
| 36 | + return ans |
| 37 | +}; |
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