阿拉凡

TL;DR

The complexity of an algorithm should be considered from two aspects:

1. Time complexity
2. Space complexity

Time and space are two directions of thinking, which is qualitative. However, if we consider the quantitative aspect, there are three directions to consider:

1. $o$ / $O$ : < / <=
2. $\omega$ / $\Omega$ : > / >=
3. $\Theta$ : =

Later, we will introduce time complexity and space complexity separately. In the next section, we will first introduce time complexity.