Trivial Runtime Analysis
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If there is no input, then it’s called a constant time algorithm. For example:

for (int i = 0; i < 1000000; i ++)
      x++;

above is O(1)
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Let’s go through some code samples and analyze their runtime complexity.

for (int i = 0; i < N; i ++)
      x++;
All we need to do is count the number of times the statement x++ will execute.
Clearly, it’s N, so the time complexity is O(N), also called linear.
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for (int i = 0; i < N; i++) 
    for (int j = 0; j < i; j++) 
        x++;
How many times the statement x++ executes:
So the time complexity is O(N^2), also called quadratic.
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Logarithmic Runtime
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Iterating powers of a number #
Let’s analyze the loop below where we iterate over all powers of 2
for (int i = 1; i <= N; i *= 2)
    x++;
    
In Big-O notation, the time complexity is O(logN)

A similar analysis gives O(logN) runtime for the loop below.
for (int i = N; i >= 1; i /= 2)
      x++;
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Harmonic series #
Consider the piece of code below:

for (int i = 1; i <= N; i++)
    for (int j = i; j <= N; j += i)
        x++;

Therefore, the time complexity is O(NlogN).
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Non Trivial Runtime
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Sum of powers #
Take the code sample below:

for (int i = 1; i <= N; i *= 2)
    for (int j = 1; j <= i; j++)
        x++;
So, the run-time complexity is actually linear - O(N)
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Amortized Analysis
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Consider this algorithm: We start with an array of size 2 
and each operation adds one element to the array, we do this operation N times. 
If the array is full, we see the current size of array say sz. 

Adding to the array if it’s not empty: O(1)
Copying array of size sz to a new location: O(sz)

Total number of operations:

1 + 1 + (1 + 2) + 1 + (1 + 4) + 1 + 1 + 1 + (1 + 8) + 1 + 1…

=> (1 + 1 + ... + 1) N times + (2 + 4 + 8 + ... ) < N + 2N<=3N

So, the complete algorithm runs in O(N) time
Allocate the 2*sz memory and copy the array to its location so we have space for the new sz elements.