Problem:

https://leetcode.com/problems/degree-of-an-array/description/

Given a non-empty array of non-negative integers nums, the degree of this array is defined as the maximum frequency of any one of its elements.

Your task is to find the smallest possible length of a (contiguous) subarray of nums, that has the same degree as nums.

Example 1:

Input: [1, 2, 2, 3, 1]
Output: 2
Explanation: 
The input array has a degree of 2 because both elements 1 and 2 appear twice.
Of the subarrays that have the same degree:
[1, 2, 2, 3, 1], [1, 2, 2, 3], [2, 2, 3, 1], [1, 2, 2], [2, 2, 3], [2, 2]
The shortest length is 2. So return 2.

Example 2:

Input: [1,2,2,3,1,4,2]
Output: 6

Solution 1: Hashtable

Time complexity: O(n)

Space complexity: O(n)

C++

// Author: Huahua
// Running time: 33 ms (beats 90.96%)
class Solution {
public:
  int findShortestSubArray(vector<int>& nums) {
    unordered_map<int, vector<int>> indices;
    for (int i = 0; i < nums.size(); ++i)
      indices[nums[i]].push_back(i);
    size_t degree = 0;
    for (const auto& p : indices)
      degree = max(degree, p.second.size());
    int ans = nums.size();
    for (const auto& p : indices) {
      if (p.second.size() != degree) continue;
      ans = min(ans, p.second.back() - p.second.front() + 1);
    }
    return ans;
  }
};

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