Solve problem 978D from codeforces

Author: AliOsm

CodeForces/978D. Almost Arithmetic Progression.cpp

@@ -0,0 +1,68 @@
+/*
+  Idea:
+    - Brute force.
+    - The first two elements in any arithmetic progression identify its factor,
+      so if we try the all possibilities for the first two elements and count
+      the number of changes each time and take the minimum we will get the
+      right answer.
+*/
+
+#include <bits/stdc++.h>
+
+using namespace std;
+
+int const N = 1e5 + 1;
+int n, a[N], b[N];
+
+int check(int chn) {
+	for(int i = 0; i < n; ++i)
+		b[i] = a[i];
+
+	int fac = b[1] - b[0];
+	for(int i = 2; i < n; ++i) {
+		if(b[i] - b[i-1] == fac)
+			continue;
+		if((b[i] - 1) - b[i-1] == fac) {
+			--b[i];
+			++chn;
+			continue;
+		}
+		if((b[i] + 1) - b[i-1] == fac) {
+			++b[i];
+			++chn;
+			continue;
+		}
+		return 1e9;
+	}
+
+	return chn;
+}
+
+int main() {
+  cin >> n;
+  for(int i = 0; i < n; ++i)
+  	cin >> a[i];
+
+  if(n <= 2) {
+  	cout << 0 << endl;
+  	return 0;
+  }
+
+  int res = 1e9;
+  res = check(0);                                         // .. ..
+  ++a[0], ++a[1];res = min(res, check(2));--a[0], --a[1]; // ++ ++
+  --a[0], --a[1];res = min(res, check(2));++a[0], ++a[1]; // -- --
+  ++a[0];res = min(res, check(1));--a[0];                 // ++ ..
+  ++a[1];res = min(res, check(1));--a[1];                 // .. ++
+  --a[0];res = min(res, check(1));++a[0];                 // -- ..
+  --a[1];res = min(res, check(1));++a[1];                 // .. --
+  --a[0], ++a[1];res = min(res, check(2));++a[0], --a[1]; // -- ++
+  ++a[0], --a[1];res = min(res, check(2));--a[0], ++a[1]; // ++ --
+
+  if(res == 1e9)
+  	cout << -1 << endl;
+  else
+  	cout << res << endl;
+
+  return 0;
+}

CodeForces/README.md

@@ -414,6 +414,7 @@
 - [978A. Remove Duplicates](http://codeforces.com/contest/978/problem/A)
 - [978B. File Name](http://codeforces.com/contest/978/problem/B)
 - [978C. Letters](http://codeforces.com/contest/978/problem/C)
+- [978D. Almost Arithmetic Progression](http://codeforces.com/contest/978/problem/D)
 - [978E. Bus Video System](http://codeforces.com/contest/978/problem/E)
 - [978F. Mentors](http://codeforces.com/contest/978/problem/F)
 - [979A. Pizza, Pizza, Pizza!!!](http://codeforces.com/contest/979/problem/A)