Add sqrt decompositon code

Author: coderick14

sqrt_decomposition.cpp

@@ -0,0 +1,84 @@
+// Solution to codeforces.com/problemset/problem/86/D using sqrt decomposition
+
+#include <bits/stdc++.h>
+#define ll long long
+#define MAX 1000001L
+using namespace std;
+
+ll block_size;
+
+struct query {
+	ll index;
+	ll start;
+	ll end;
+    ll ans;
+};
+
+bool comp(query a, query b) {
+	if(a.start/block_size != b.start/block_size) {
+		return a.start/block_size < b.start/block_size;
+	}
+	return a.end < b.end;
+}
+
+bool compIndex(query a, query b) {
+    return a.index < b.index;
+}
+
+int main() {
+	ll i,n,t;
+    cin>>n>>t;
+	ll a[n+1];
+    vector<long> freq(MAX);
+    for(i=0; i<MAX; i++)
+    	freq[i] = 0;
+	vector<query> queries(t);
+	block_size = int(sqrt(n)) + 1;
+	for(i=1; i<=n; i++) {
+		cin>>a[i];
+	}
+	for(i=0; i<t; i++) {
+		cin>>queries[i].start>>queries[i].end;
+		queries[i].index = i;
+        queries[i].ans = 0;
+	}
+	sort(queries.begin(), queries.end(), comp);
+    ll currentL = queries[0].start, currentR = queries[0].end;
+    for(i=currentL; i<=currentR; i++) {
+        queries[0].ans += (2*freq[a[i]] + 1)*a[i];
+        freq[a[i]]++;
+    }
+
+    for(i=1; i<t; i++) {
+        queries[i].ans = queries[i-1].ans;
+
+        // invrease window first
+        while(currentL > queries[i].start) {
+            currentL--;
+            queries[i].ans += (2*freq[a[currentL]] + 1)*a[currentL];
+            freq[a[currentL]]++;
+        }
+        while(currentR < queries[i].end) {
+            currentR++;
+            queries[i].ans += (2*freq[a[currentR]] + 1)*a[currentR];
+            freq[a[currentR]]++;
+        }
+
+        // decrease window
+        while(currentL < queries[i].start) {
+            freq[a[currentL]]--;
+            queries[i].ans -= (2*freq[a[currentL]] + 1)*a[currentL];
+            currentL++;
+        }
+        while(currentR > queries[i].end) {
+            freq[a[currentR]]--;
+            queries[i].ans -= (2*freq[a[currentR]] + 1)*a[currentR];
+            currentR--;
+        }
+    }
+    sort(queries.begin(), queries.end(), compIndex);
+    for(i=0; i<t; i++) {
+        cout<<queries[i].ans<<"\n";
+    }
+	return 0;
+}