【链接】 我是链接,点我呀:)
【题意】

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());
        }
    }
}