Creating Hashing Directory with one problem

Author: bishopmate

Hashing/sequence_pair_weight.cpp

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+/*
+Problem Link - https://codeforces.com/contest/1527/problem/C
+
+Problem Statement - The weight of a sequence is defined as the number of unordered pairs of indexes (i,j)
+(here i<j) with same value (ai=aj). For example, the weight of sequence a=[1,1,2,2,1] is 4. The set of unordered pairs of indexes with same value are (1,2), (1,5), (2,5), and (3,4).
+
+You are given a sequence a of n integers. Print the sum of the weight of all subsegments of a.
+A sequence b is a subsegment of a sequence a if b can be obtained from a by deletion of several (possibly, zero or all) elements from the beginning and several (possibly, zero or all) elements from the end.
+
+*/
+
+#include<bits/stdc++.h>
+#define int long long
+
+using namespace std;
+
+void solve(int n, vector<int> v){
+  int ans = 0;
+  map<int, int> toLeft;
+  for(int i=1; i<=n; i++){
+    if(toLeft[v[i]] != 0){
+      int toRight = n-i+1;
+      ans += toRight*toLeft[v[i]];
+    }
+    toLeft[v[i]] += i;
+  }
+  cout<<ans<<"\n";
+}
+
+signed main(){
+  ios::sync_with_stdio(false);cin.tie(0);
+  int t   = 1;
+  cin >> t;
+  for(int i=1; i<=t; i++){           
+    int n;
+    cin>>n;
+    vector<int> v(n+1);
+    for(int j=1; j<=n; j++){
+      cin>>v[j];
+    }
+    solve(n,v);
+  }
+  return 0;
+}
+
+// Tutorial Link to understand the solution - https://codeforces.com/blog/entry/90939