697. Degree of an Array

Love The Way You Lie 2022-06-03 03:12 109阅读 0赞

> 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.

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这道题整整花了快一个小时，试了好几种方法。先来分析题目，找到频率最大，距离最小的子数组的长度，根据上一道题的经验，只遍历一遍肯定是最优的解法，所以一开始我想的是用map寻找出频率最高的，再使用遍历数组找出元素的长度，但还是想用一种更为简便的方法去解，一般都是用空间去换时间。

这里声明了两个map，一个存储元素第一次出现的下标，第二个map存储出现的次数，通过`len = min(i - map[nums[i]] + 1, len);`来取出现频率相同的子数组中的最小长度，最后返回len

//
    // Created by will on 2017/12/15.
    //
    
    
    /* * 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. */
    
    
    #include <iostream>
    #include <vector>
    #include <unordered_map>
    
    using namespace std;
    
    
    class Solution {
    public:
        int findShortestSubArray(vector<int>& nums) {
            unordered_map<int, int> map,count;
    
            int len = nums.size(), flag = 0;
    
            for (int i = 0; i < nums.size(); ++i) {
                if (map.count(nums[i]) == 0) map[nums[i]] = i;
                count[nums[i]]++;
    
                if(count[nums[i]] == flag){
                    len = min(i - map[nums[i]] + 1, len);
                }
    
                if (count[nums[i]] > flag){
                    len = i - map[nums[i]] + 1, len;
                    flag = count[nums[i]];
                }
    
            }
    
            return len;
    
    
    
    
        }
    };
    
    
    
    int main(){
        vector<int> nums = {
       1,2,3,4,2};
        Solution s;
        int len = s.findShortestSubArray(nums);
        cout<<len;
    
        return 0;
    }

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*  Runtime: 59 ms
 *  时间复杂度：O(n)；空间复杂度：O(1)