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C++ code to find index where this is a hash collision
Suppose we have a number p and another array X with n elements. There is a hash table with p buckets. The buckets are numbered from 0 to p-1. We want to insert n numbers from X. We are assuming for X[i], its bucket will be selected by the hash function h(X[i]), where h(k) = k mod p. One bucket cannot hold more than one element. If We want to insert an number into a bucket which is already filled, we say a "collision" happens. We have to return the index where collision has happened. If there is no collision, return -1.
So, if the input is like p = 10; X = [0, 21, 53, 41, 53], then the output will be 3, because first one will be inserted into 0, second one at 1, third one at 3, but 4th one will try to insert into 1 but there is a collision, the index is 3 here.
Steps
To solve this, we will follow these steps −
n := size of X Define an array arr of size: p and fill with 0 for initialize i := 0, when i < n, update (increase i by 1), do: x := X[i] if arr[x mod p] is non-zero, then: return i increase arr[x mod p] by 1 return -1
Example
Let us see the following implementation to get better understanding −
#include <bits/stdc++.h> using namespace std; int solve(int p, vector<int> X){ int n = X.size(); int arr[p] = { 0 }; for (int i = 0; i < n; i++){ int x = X[i]; if (arr[x % p]){ return i; } arr[x % p]++; } return -1; } int main(){ int p = 10; vector<int> X = { 0, 21, 53, 41, 53 }; cout << solve(p, X) << endl; }
Input
10, { 0, 21, 53, 41, 53 }
Output
3
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