2364. Count Number of Bad Pairs

Medium

You are given a 0-indexed integer array nums. A pair of indices (i, j) is a bad pair if i < j and j - i != nums[j] - nums[i].

Return the total number of bad pairs in nums.

Example 1:

Input: nums = [4,1,3,3]
Output:
 5
Explanation:
 The pair (0, 1) is a bad pair since 1 - 0 != 1 - 4.
The pair (0, 2) is a bad pair since 2 - 0 != 3 - 4, 2 != -1.
The pair (0, 3) is a bad pair since 3 - 0 != 3 - 4, 3 != -1.
The pair (1, 2) is a bad pair since 2 - 1 != 3 - 1, 1 != 2.
The pair (2, 3) is a bad pair since 3 - 2 != 3 - 3, 1 != 0.
There are a total of 5 bad pairs, so we return 5.

Example 2:

Input: nums = [1,2,3,4,5]
Output:
 0
Explanation:
 There are no bad pairs.

Constraints:

• 1 <= nums.length <= 10^5

• 1 <= nums[i] <= 10^9

解題

Runtime: 119 ms, faster than 100%

Memory Usage: 14.1 MB, less than 16.67%

func countBadPairs(nums []int) int64 {
    m := make(map[int]int)
    res := (len(nums) * (len(nums) - 1))/2 // 所有可能 C len(nums) 取 2

    for i, n := range nums {
        m[n - i]++ // 紀錄 nums[i] - i 相同的有多少組
    }

    for k := range m {
        res -= (m[k]*(m[k] - 1))/2 // 扣去 not bad pair 總數
    }

    return int64(res)
}