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What is Time Complexity? What are the different types of Time Complexity



The Time Complexity is the amount of time taken by an algorithm to run, as a function of the length of the input. Here, the length of input indicates the number of operations to be performed by the algorithm. This gives a clear indication of what exactly Time complexity tells us. It is not going to examine the total execution time of an algorithm. Rather, it is going to give information about the variation (increase or decrease) in execution time when the number of operations (increase or decrease) in an algorithm. Yes, as the definition says, the amount of time taken is a function of the length of input only.


To elaborate, Time complexity measures the time taken to execute each statement of code in an algorithm. If a statement is set to execute repeatedly then the number of times that statement gets executed is equal to N multiplied by the time required to run that function each time.


What are the different types of Time complexity notation used?

As we have seen, Time complexity is given by time as a function of the length of the input. And, there exists a relation between the input data size (n) and a number of operations performed (N) with respect to time. This relation is denoted as Order of growth in Time complexity and given notation O[n] where O is the order of growth and n is the length of the input. It is also called as ‘Big O Notation’


Big O Notation expresses the run time of an algorithm in terms of how quickly it grows relative to the input ‘n’ by defining the N number of operations that are done on it. Thus, the time complexity of an algorithm is denoted by the combination of all O[n] assigned for each line of function.


There are different types of time complexities used, let’s see one by one:

1. Constant time – O (1)

2. Linear time – O (n)

3. Logarithmic time – O (log n)

4. Quadratic time – O (n^2)

5. Cubic time – O (n^3)


and many more complex notations like Exponential time, Quasilinear time, factorial time, etc. are used based on the type of functions defined.


1. Constant time – O (1)

An algorithm is said to have constant time with order O (1) when it is not dependent on the input size n. Irrespective of the input size n, the runtime will always be the same. Example as shown:



The above code shows that irrespective of the length of the array (n), the runtime to get the first element in an array of any length is the same. If the run time is considered as 1 unit of time, then it takes only 1 unit of time to run both the arrays, irrespective of length. Thus, the function comes under constant time with order O (1).


2. Linear time – O(n)

An algorithm is said to have a linear time complexity when the running time increases linearly with the length of the input. When the function involves checking all the values in input data, such function has Time complexity with this order O(n). For example:



The above code shows that based on the length of the array (n), the run time will get linearly increased. If the run time is considered as 1 unit of time, then it takes only n times 1 unit of time to run the array. Thus, the function runs linearly with input size and this comes with order O(n).


3. Logarithmic time – O (log n)

An algorithm is said to have a logarithmic time complexity when it reduces the size of the input data in each step. This indicates that the number of operations is not the same as the input size. The number of operations gets reduced as the input size increases. Algorithms with Logarithmic time complexity are found in binary trees or binary search functions. This involves the search of a given value in an array by splitting the array into two and starting searching in one split. This ensures the operation is not done on every element of the data.


4. Quadratic time – O (n^2)

An algorithm is said to have a non – linear time complexity where the running time increases non-linearly (n^2) with the length of the input. Generally, nested loops come under this time complexity order where for one loop takes O(n) and if the function involves a loop within a loop, then it goes for O(n)*O(n) = O(n^2) order.

Similarly, if there are ‘m’ loops defined in the function, then the order is given by O (n ^ m), which are called polynomial time complexity functions.

The order of growth for all time complexities are indicated in the graph below:



Thus, the above illustration gives a fair idea of how each function gets the order notation based on the relation between run time against the number of input data size and number of operations performed on them.


Time Complexity of Sorting algorithms

Understanding the time complexities of sorting algorithms helps us in picking out the best sorting technique in a situation. Here are the time complexities of some sorting techniques:


1. Time Complexity of Insertion Sort:

The time complexity of Insertion Sort in the best case is O(n). In the worst case, the time complexity is O(n^2).


2. Time Complexity of Merge Sort:

This sorting technique has a stable time complexity for all kinds of cases. The time complexity of Merge Sort in the best case is O(nlogn). In the worst case, the time complexity is O(nlogn). This is because Merge Sort implements a same number of sorting steps for all kinds of cases.


3. Time Complexity of Bubble Sort:

The time complexity of Bubble Sort in the best case is O(n). In the worst case, the time complexity is O(n^2).


4. Time Complexity of Quick Sort:

The time complexity of Quick Sort in the best case is O(nlogn). In the worst case, the time complexity is O(n^2). Quicksort is considered to be the fastest of the sorting algorithms due to its performance of O(nlogn) in best and average cases.


Time Complexity of Searching algorithms

Let us now dive into the time complexities of some Searching Algorithms and understand which of them is faster.


1. Time Complexity of Linear Search:

Linear Search follows the sequential access. The time complexity of Linear Search in the best case is O(1). In the worst case, the time complexity is O(n).


2. Time Complexity of Binary Search:

Binary Search is the faster of the two searching algorithms. However, for smaller arrays, linear search does a better job. The time complexity of Binary Search in the best case is O(1). In the worst case, the time complexity is O(log n).




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