README.md

2364. Count Number of Bad Pairs

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 <= 105
• 1 <= nums[i] <= 109

Solutions (Rust)

1. Solution

use std::collections::HashMap;

impl Solution {
    pub fn count_bad_pairs(nums: Vec<i32>) -> i64 {
        let mut count = HashMap::new();
        let mut ret = 0;

        for i in 0..nums.len() {
            ret += i as i64 - count.get(&(i as i32 - nums[i])).unwrap_or(&0);
            count
                .entry(i as i32 - nums[i])
                .and_modify(|c| *c += 1)
                .or_insert(1);
        }

        ret
    }
}