Computing a raised to b mod c in logn time

fast_power.cpp

@@ -0,0 +1,31 @@
+#include<bits/stdc++.h>
+#define MOD 1000000007
+#define MOD9 1000000009
+#define forr(i,p,n) for(ll i=p;i<n;i++)
+#define endl "\n"
+typedef long long ll;
+using namespace std;
+
+ll fast_power(ll n, ll k, ll p = MOD) {
+ll r = 1; 
+  for (; k; k >>= 1)
+   {
+    if (k & 1) 
+    {
+        r = r * n%p; 
+    }
+    n = n * n%p;
+   } 
+return r;
+ }
+
+
+int main(){
+    ios::sync_with_stdio(0);
+    cin.tie(0);cout.tie(0);
+     ll a,b,c;
+     cin>>a>>b>>c;
+     ll ans=fast_power(a,b,c);
+     cout<<ans<<endl;
+      return 0;  
+}