# Code

The solution of problem 982 is showed below. You can also find it here.

1 | class Solution { |

# Solution

The core method to solve it is to treat one number previously and traverse all combination of two numbers $a \& b$ in the vector. In this way, we can reduce the time complexity from $O(N^3)$ to $O(N^2)$. The treatment is to pre-calculate each number $a$ in the vector to find with which number $b$ $a \& b$ would get $0$.

# Valuable Method

The pre-treating method is really important when solving problems. If we enumerate all three numbers combination, for example $a \& b \& c$, we would repeatly calculate & operator with $c$ for many times. By remembering it, we could use more space to reduce run time. Moreoever, we could also store the result of $a \& b$ and check the result by $c$.