【Codeforces 466C】Number of Ways

让你把数组分成3个连续的部分
每个部分的和要一样
问你有多少种分法

【题解】

先处理出来num[i]
表示i..n这里面有多少个j
满足aft[j] = aft[i]/2
这aft[i]=a[j]+a[j+1]..+a[n]
然后for从1..n
看看pre[i]*2=aft[i+1]是否成立。
如果成立的话那么答案就加上num[i+1]

【代码】

import java.io.*;

import java.util.*;

public class Main {

	static int N = (int)5e5;

	static InputReader in;

	static PrintWriter out;

	public static void main(String[] args) throws IOException{

		in = new InputReader();

		out = new PrintWriter(System.out);

		//code start from here

		new Task().solve(in, out);

		out.close();

	}

	static class Task{

		public void solve(InputReader in,PrintWriter out) {

			int n = in.nextInt();

			int []a = new int[N+10];

			long []pre = new long[N+10];

			long []aft = new long[N+10];

			int []num = new int[N+10];

			for (int i = 1;i <= n;i++) a[i] = in.nextInt();

			for (int i = 1;i <= n;i++) pre[i] = pre[i-1]+a[i];

			for (int i = n;i >= 1;i--) aft[i] = aft[i+1]+a[i];

			Hashtable<Long, Integer> dic = new Hashtable<Long,Integer>();

			for (int i = n;i >= 1;i--) {

				if (aft[i]%2==0) {

					long temp = aft[i]/2;

					if (dic.get(temp)!=null)

						num[i] = dic.get(temp);

				}

				Integer temp1 = dic.get(aft[i]);

				if (temp1==null) {

					dic.put( aft[i], 1);

				}else {

					dic.put(aft[i], temp1+1);

				}

			}

			long ans = 0;

			for (int i = 1;i <=n;i++) {

				long cur = pre[i];

				cur = pre[n]-cur;

				if (cur!=pre[i]*2) {

					continue;

				}

				ans = ans + num[i+1];

			}

			out.println(ans);

		}

	}

	static class InputReader{

		public BufferedReader br;

		public StringTokenizer tokenizer;

		public InputReader() {

			br = new BufferedReader(new InputStreamReader(System.in));

			tokenizer = null;

		}

		public String next(){

			while (tokenizer==null || !tokenizer.hasMoreTokens()) {

				try {

				tokenizer = new StringTokenizer(br.readLine());

				}catch(IOException e) {

					throw new RuntimeException(e);

				}

			}

			return tokenizer.nextToken();

		}

		public int nextInt() {

			return Integer.parseInt(next());

		}

	}

}

巴特西

【Codeforces 466C】Number of Ways

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