Described : you have an array: arr = [ [0 , 3] , [2 , 5] , [0 , 9] ] Ex-: here in first element or arr[ 0 ] is [0, 3] will be [0=’A’ , time=3] similarly- arr[ 0 ]= [ ‘A’ , time = 3 ] arr[ 1 ]= [ ‘C’ , time = 5–3 = 2 ] arr[ 2 ]= [ ‘A’ , time = 9–5 = 4 ] So, arr of index 2 is taking more time so the key ‘A’ is slower than others. Your stdout/return should be ‘A’