no need to two variables to calculate result. In Java public static int diagonalDifference(List> arr) { int n = arr.size(); int result =0; for (int i = 0; i < n; i++) { result += arr.get(i).get(i) - arr.get( i).get(n - i -1); } return Math.abs(result); }

in C

int diagonalDifference(int arr_rows, int arr_columns, int** arr) {
    
    int LTRsum = 0, RTLsum = 0;
    
    for(int r=0,c=0;r<arr_rows && c<arr_columns;r++,c++){
                    
                /*LTR*/
                LTRsum+=arr[r][c];
                /*RTL*/
                RTLsum+= arr[r][arr_columns-1-c];
            
    }
    
    if((RTLsum-LTRsum) >= 0){
        return RTLsum-LTRsum;
    }else {
    return  LTRsum-RTLsum;
    }
}

Draw a 3x3 matrix with a pen and paper, and mark out the corrdinates for each diagonal. Will make it clearer.

I had a slightly nuanced approach to what most people did, thought I'd share. Python

def diagonalDifference(arr):
    length = len(arr)
    diff = 0
    for i in range(0, length):
        diff = diff - (arr[i][i] - arr[i][length-(1+i)])
        
    return abs(diff)

Java 8:

public static int diagonalDifference(List<List<Integer>> arr) {
int sum1 = 0;
int sum2 = 0;
    int j = arr.size() - 1;
    for (int i = 0; i < arr.size(); i++) {
        sum1 += arr.get(i).get(i);
        sum2 += arr.get(i).get(j);
        j--;
    }
    return Math.abs(sum1 - sum2);
}