# A Time Complexity Question

What is the time complexity of following function fun()? Assume that log(x) returns log value in base 2.

`void` `fun() ` `{ ` ` ` `int` `i, j; ` ` ` `for` `(i=1; i<=n; i++) ` ` ` `for` `(j=1; j<=` `log` `(i); j++) ` ` ` `printf` `(` `"GeeksforGeeks"` `); ` `} ` |

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Time Complexity of the above function can be written as Θ(log 1) + Θ(log 2) + Θ(log 3) + . . . . + Θ(log n) which is Θ (log n!)

Order of growth of ‘log n!’ and ‘n log n’ is same for large values of n, i.e., Θ (log n!) = Θ(n log n). So time complexity of fun() is Θ(n log n).

The expression Θ(log n!) = Θ(n log n) can be easily derived from following Stirling’s approximation (or Stirling’s formula).

log n! = n log n - n + O(log(n))

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

Sources:

http://en.wikipedia.org/wiki/Stirling%27s_approximation

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