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2615 - Sum of Distances (Medium)

https://leetcode.com/problems/sum-of-distances/

Problem Statement​

You are given a 0-indexed integer array nums. There exists an array arr of length nums.length, where arr[i] is the sum of |i - j| over all j such that nums[j] == nums[i] and j != i. If there is no such j, set arr[i] to be 0.

Return the arrayarr.

Example 1:

Input: nums = [1,3,1,1,2]
Output: [5,0,3,4,0]
Explanation:
When i = 0, nums[0] == nums[2] and nums[0] == nums[3]. Therefore, arr[0] = |0 - 2| + |0 - 3| = 5.
When i = 1, arr[1] = 0 because there is no other index with value 3.
When i = 2, nums[2] == nums[0] and nums[2] == nums[3]. Therefore, arr[2] = |2 - 0| + |2 - 3| = 3.
When i = 3, nums[3] == nums[0] and nums[3] == nums[2]. Therefore, arr[3] = |3 - 0| + |3 - 2| = 4.
When i = 4, arr[4] = 0 because there is no other index with value 2.

Example 2:

Input: nums = [0,5,3]
Output: [0,0,0]
Explanation: Since each element in nums is distinct, arr[i] = 0 for all i.

Constraints:

  • 1 <= nums.length <= 10^5
  • 0 <= nums[i] <= 10^9

Approach 1: Prefix Sum​

Written by @wkw
class Solution {
public:
    vector<long long> distance(vector<int>& nums) {
        unordered_map<int, vector<int>> m;
        vector<long long> ans(nums.size());
        int n = nums.size();
        // for each number x, collect all the indices where x occurs
        for (int i = 0; i < n; i++) m[nums[i]].push_back(i);
        for (auto x : m) {
            // calcualte the prefix sum of the array
            vector<int> v = x.second;
            int n = v.size();
            vector<long long> pre(n + 1);
            for (int i = 0; i < n; i++) pre[i + 1] = pre[i] + v[i];
            // for each occurrence of x,
            // the indices to the right will be regular subtraction
            // while the indices to the left will be reversed subtraction.
            for (int i = 0; i < n; i++) {
                long long k = v[i];
                ans[k] = (k * (i + 1) - pre[i + 1]) +
                            (pre[n] - pre[i] - k * (n - i));
            }
        }
        return ans;
    }
};