A question about for-loop condition in java

Question

I am working on the problem below: Given a non-empty array of digits representing a non-negative integer, plus one to the integer. Eg:[1,2,3] Output:[1,2,4] My approach is usring carry to calculate the singular digit from end to start. I got 2 version of this problem. First one does not work when the input is [1,4,4,9].However, the second one is wokring. In the second version, I change the for-loop condition. I wonder why this happened? Could anyone help me with this? Any help would be appreciated! Thanks! Version 1:

   public static int[] plusOne(int[] digits) {
        int sum = 0;
        int len = digits.length;
        int carry = (digits[digits.length -1]+1)/10;
        digits[digits.length -1] = (digits[digits.length -1]+1)%10;

        for (int j = len-2;j >= 0;j--){

            carry = (digits[j]+carry)/10 ;
            digits[j] =(digits[j]+carry)%10 ;
        }

        if (carry==0) return digits;
        else{
            int[] res = new int[len+1];
            res[0] = 1;
            int i = 0;
            for (int j=1;j<res.length;j++){
                res[j] = digits [i++];
            }
            return res;
        }
    }

Version 2:

public static int[] plusOne(int[] digits) {
        int sum = 0;
        int len = digits.length;
        int carry = (digits[digits.length -1]+1)/10;
        digits[digits.length -1] = (digits[digits.length -1]+1)%10;

        for (int j = len-2;j >= 0;j--){
        int temp = digits[j]+carry;
        carry = (temp)/10 ;
        digits[j] =(temp)%10 ;
        }

        if (carry==0) return digits;
        else{
            int[] res = new int[len+1];
            res[0] = 1;
            int i = 0;
            for (int j=1;j<res.length;j++){
                res[j] = digits [i++];
            }
            return res;
        }

Answer 1

_{Posting a longer answer after all...}

The only difference in those two code snippets are the first for loops. Your first example doesn't work as you expected.

for (int j = len-2;j >= 0;j--){
        carry = (digits[j]+carry)/10 ;
        digits[j] =(digits[j]+carry)%10 ;
}

Here the problem lies in the line carry = (digits[j]+carry)/10 ;
You're changing the variable carry , but this value is being used in the next line.

Let's say, carry was 1 and digits[j] is 5 . After carry = (digits[j]+carry)/10; the variable carry changed to 0 and the subsequent calculation of digits[j] results in 5 instead of 6 .

The second example works, because you're introducing an intermediate variable temp :

for (int j = len-2;j >= 0;j--){
    int temp = digits[j]+carry;
    carry = (temp)/10 ;
    digits[j] =(temp)%10 ;
}

temp is calculated once and then only read from, so carry and digits are not dependant on each other any more. In turn, you will get the result you expect.