My first solved-problem on Codeforces. It's really an easy problem. As we know, if you want to make the a and b as small as possible, they must only contain digits 0 and 1. So, we have the thinking in the following: 1. if the digit of x[i] is 2: we try to make a[i] and b[i] both 1. 2. if the digit of x[i] is 0: we try to make a[i] and b[i] both 0. 3. if the digit of x[i] is 1: either a[i] or b[i] would be 1. We'll make a[i] 1 because it is no difference if we make b[i] 1. However, if we make a[i] 1, the number a must be larger than b as a fact. So, we would not want to make array a still bigger. So after we make a[i] 1, we have different thinking: 1. if the digit of x[i] is 2: we make a[i] 0 and b[i] 2. 2. if the digit of x[i] is 0: we make a[i] and b[i] both 0. 3. if the digit of x[i] is 1: we make a[i] 0 and b[i] 1. That's all. We'll discover that after we make a[i] 1, a does not increase anymore, but b is increasing, however it will never be bigger than a. Thanks for reading, hope that'll help you!