题解 | 喜欢切数组的红

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        if (scanner.hasNextInt()) {
            int n = scanner.nextInt();
            int[] a = new int[n];
            for (int i = 0; i < n; i++) {
                a[i] = scanner.nextInt();
            }

            // 1. 预处理前缀和与正数计数
            long[] prefixSum = new long[n + 1];
            int[] posPrefix = new int[n + 1];

            for (int i = 1; i <= n; i++) {
                prefixSum[i] = prefixSum[i - 1] + a[i - 1];
                posPrefix[i] = posPrefix[i - 1] + (a[i - 1] > 0 ? 1 : 0);
            }

            long totalSum = prefixSum[n];

            if (totalSum % 3 != 0) {
                System.out.println(0);
                scanner.close();
                return;
            }

            long target = totalSum / 3;
            long ans = 0;
		  
		  	//使用 Map存储前缀和出现的次数
            // Key: 正数前缀和的值 (pos_prefix[j])
            // Value: 该值出现的次数 (有多少个合法的 j)
            Map<Integer, Integer> countMap = new HashMap<>();

            // 3. 从右向左扫描
            for (int i = n - 2; i >= 1; i--) {
                // --- A. 将 i+1 加入 Map ---
                int j = i + 1;
                if (j <= n - 1) {
                    boolean isValidJSum = (prefixSum[j] == 2 * target);
                    boolean hasPositivePart3 = (posPrefix[n] - posPrefix[j] > 0);
                    
                    if (isValidJSum && hasPositivePart3) {
                        int key = posPrefix[j];
                        // 计数加 1
                        countMap.put(key, countMap.getOrDefault(key, 0) + 1);
                    }
                }

                // --- B. 查询 Map ---
                
                if (prefixSum[i] == target && posPrefix[i] > 0) {
                    int currentPosCount = posPrefix[i];
                    
                    // 核心逻辑：统计 Map 中所有 Key > currentPosCount 的 Value 之和
                    int countValidJ = 0;
                    for (Map.Entry<Integer, Integer> entry : countMap.entrySet()) {
                        int mapKey = entry.getKey();
                        int mapValue = entry.getValue();
                        
                        if (mapKey > currentPosCount) {
                            countValidJ += mapValue;
                        }
                    }
                    ans += countValidJ;
                }
            }

            System.out.println(ans);
        }
        scanner.close();
    }
}