Time complexity is one of the most important concepts in competitive programming because it determines how fast your algorithm runs relative to the size of the input. In contests, problems usually have constraints on the size of the input, and your solution must run within a specified time limit, often one or two seconds. If your algorithm's time complexity is too high for the input size, you'll encounter time-limit exceeded (TLE) errors, meaning your solution took too long to execute. The most common time complexities you'll encounter are O(1), O(log n), O(n), O(n log n), O(n^2), and O(2^n). For example, an O(n log n) algorithm like merge sort is efficient enough to handle input sizes up to 100,000 elements, while an O(n^2) algorithm like bubble sort would struggle with the same input size. Understanding the relationship between input size and time complexity allows you to choose or design algorithms that scale well, even for large inputs. Additionally, competitive programming often requires you to optimize solutions to fit within both time and space limits. By analyzing time complexity, you can identify parts of your algorithm that are inefficient and look for ways to improve them, such as using more advanced data structures like heaps or segment trees. Practicing problems with different time complexities will help you develop an intuition for recognizing which algorithms to apply based on the input constraints.