ThreeEqualParts.java

class Solution {
  public int[] threeEqualParts(int[] arr) {
    int[] ans = new int[] {-1,-1};

    // num Of 1
    int numOf1s = 0;
    for (int el : arr) {
      numOf1s += el;
    }

    if (numOf1s == 0) {
      return new int[]{0, arr.length-1};
    }

    if (numOf1s % 3 != 0) {
      return ans;
    }

    int noOfOnesInEachPart = numOf1s / 3;
    int indexOfFirst1InPart0 = -1;
    int indexOfFirst1InPart1 = -1;
    int indexOfFirst1InPart2 = -1;
    numOf1s = 0;
    for (int i = 0; i < arr.length; i++) {
      if (arr[i] == 1) {
        numOf1s++;
        if (numOf1s == noOfOnesInEachPart + 1) {
          indexOfFirst1InPart1 = i;
        } else if (numOf1s == 2 * noOfOnesInEachPart + 1) {
          indexOfFirst1InPart2 = i;
        } else if (numOf1s == 1) {
          indexOfFirst1InPart0 = i;
        }
      }
    }

    while (indexOfFirst1InPart2 < arr.length) {
      if (arr[indexOfFirst1InPart2] == arr[indexOfFirst1InPart0] && arr[indexOfFirst1InPart2] == arr[indexOfFirst1InPart1]) {
        indexOfFirst1InPart2++;
        indexOfFirst1InPart1++;
        indexOfFirst1InPart0++;
      } else {
        return ans;
      }
    }

    return new int[]{indexOfFirst1InPart0 - 1, indexOfFirst1InPart1};
  }
}