PlusOne.java

package LeetCodeJava.Array;

// https://leetcode.com/problems/plus-one/

public class PlusOne {

    // V0

    // V1
    // https://leetcode.com/problems/plus-one/editorial/
    public int[] plusOne(int[] digits) {
        int n = digits.length;

        // move along the input array starting from the end
        for (int idx = n - 1; idx >= 0; --idx) {
            // set all the nines at the end of array to zeros
            if (digits[idx] == 9) {
                digits[idx] = 0;
            }
            // here we have the rightmost not-nine
            else {
                // increase this rightmost not-nine by 1
                digits[idx]++;
                // and the job is done
                return digits;
            }
        }
        // we're here because all the digits are nines
        digits = new int[n + 1];
        digits[0] = 1;
        return digits;
    }

    // V2
    // https://leetcode.com/problems/plus-one/solutions/2706861/java-fastest-0ms-runtime-easy-and-elegant-solution/
    public int[] plusOne_2(int[] digits){
        for (int i = digits.length - 1; i >= 0; i--) {
            if (digits[i] < 9) {
                digits[i]++;
                return digits;
            }
            digits[i] = 0;
        }

        digits = new int[digits.length + 1];
        digits[0] = 1;
        return digits;
    }

}