三等分

/*
 * Problem Statement:
 * Given an array arr of 0s and 1s, divide the array into 3 non-empty parts such that all of these parts represent the same binary value.
 * If possible, return any [i, j] with i + 1 < j, such that:
 * - arr[0], arr[1], ..., arr[i] is the first part;
 * - arr[i + 1], arr[i + 2], ..., arr[j - 1] is the second part;
 * - arr[j], arr[j + 1], ..., arr[arr.length - 1] is the third part.
 * These parts must represent the same binary value.
 * If it's not possible, return [-1, -1].
 *
 * Example:
 * Input: arr = [1,0,1,0,1]
 * Output: [0, 3]
 *
 * Approach:
 * 1. Count the total number of 1s in the array.
 * 2. If this number is not divisible by 3, return [-1, -1] since we cannot split them equally.
 * 3. Identify the positions of the 1s and attempt to divide them into three equal parts.
 * 4. Verify that the segments between these positions represent the same binary value.
 */

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class Solution {
    public int[] threeEqualParts(int[] arr) {
        long countOnes = Arrays.stream(arr).filter(i -> i == 1).count();
        if (countOnes % 3 != 0) return new int[]{-1, -1};
        if (countOnes == 0) return new int[]{0, arr.length - 1};

        long k = countOnes / 3;
        List<Integer> positions = Arrays.stream(arr).boxed().collect(Collectors.toList());
        int first = positions.indexOf(1);
        int second = nthIndexOf(arr, 1, k + 1);
        int third = nthIndexOf(arr, 1, 2 * k + 1);

        while (third < arr.length && arr[first] == arr[second] && arr[second] == arr[third]) {
            first++;
            second++;
            third++;
        }

        if (third == arr.length) return new int[]{first - 1, second};
        return new int[]{-1, -1};
    }

    private int nthIndexOf(int[] arr, int value, long n) {
        int index = -1;
        for (int i = 0; i < arr.length; i++) {
            if (arr[i] == value) {
                n--;
                if (n == 0) {
                    index = i;
                    break;
                }
            }
        }
        return index;
    }
}