Leetcode-1512. Number of good pairs

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1512. A good number of pairs

subject

Give you an array of integers nums .

If a set of numbers (i,j) Satisfy nums[i] == nums[j] And i < j , You can think of this as a group Good number of pairs .

Returns the number of good pairs .

Example

Example 1：
Input ：nums = [1,2,3,1,1,3]
Output ：4
explain ： Yes 4 Group several pairs , Namely (0,3), (0,4), (3,4), (2,5) , Subscript from 0 Start

Example 2：
Input ：nums = [1,1,1,1]
Output ：6
explain ： Each group of numbers in the array is a good number pair

Example 3：
Input ：nums = [1,2,3]
Output ：0

explain

• 1 <= nums.length <= 100
• 1 <= nums[i] <= 100

Ideas

Let's give you an example ,[1,1,1], For this array , When traversing to the subscript 1 Of 1 When , Can be subscripted with 0 Of 1 Form a pair of answers , When traversing to the subscript 2 Of 1 When , There are two in front 1, Therefore, it can form 2 The answer .

So we can traverse the array while , Use a hash table to record the current number of each element , How many elements of this element have appeared before , How many pairs can be added to the answer .

C++ Code

class Solution {
public:
    int numIdenticalPairs(vector<int>& nums) {
        unordered_map <int, int> map;
        int ans = 0;
        for (int num: nums)
        {
            ans+=map[num];
            map[num]++;
        }
        return ans;
    }
};