Given an array of integers arr, write a function that returns true if and only if the number of occurrences of each value in the array is unique.

Example 1:

Input: arr = [1,2,2,1,1,3]
Output: true
Explanation: The value 1 has 3 occurrences, 2 has 2 and 3 has 1. No two values have the same number of occurrences.

Example 2:

Input: arr = [1,2]
Output: false

Example 3:

Input: arr = [-3,0,1,-3,1,1,1,-3,10,0]
Output: true

Constraints:

• 1 <= arr.length <= 1000
• -1000 <= arr[i] <= 1000

这道题给了一个整型数组，问每个数字出现的次数都否都不同。对于一道 Easy 的题目来说，没有太大的难度，就是用个 HashMap 来统计每个数字出现的次数，然后再用个 HashSet 来判断某个次数是否之前出现过了，若出现过了，则返回 false，否则最终返回 true 即可，参见代码如下：

class Solution {
public:
    bool uniqueOccurrences(vector<int>& arr) {
        unordered_map<int, int> numCnt;
        unordered_set<int> st;
        for (int num : arr) ++numCnt[num];
        for (auto a : numCnt) {
            if (st.count(a.second)) return false;
            st.insert(a.second);
        }
        return true;
    }
};

Github 同步地址:

https://github.com/grandyang/leetcode/issues/1207

参考资料：

https://leetcode.com/problems/unique-number-of-occurrences/

https://leetcode.com/problems/unique-number-of-occurrences/discuss/392858/JavaPython-3-4-liner-and-2-liner-Using-Map-and-Set-w-brief-explanation-and-analysis.

LeetCode All in One 题目讲解汇总(持续更新中...)