personal-notes.md

personal notes

i forget stuffs a lot so i'lll write any interesting notable things here.

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converting a charchter to ascii

any char will be converted to it's ascii representation, on doing int().

char c = 'a';
int i = int(c); // i = 97

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converting a number to characters.

ie. convert (int)7 to (char)'7'.

int i = 6;
char c = i + '0'; // important step
cout << c;
// this works or 0-9.

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Storing Negative numbers in C++

We convertthe number to it's 2's compliment and then store it.

// first bit = 0 ~ POSITIVE NUMBER
// first bit = 1 ~ NEGATIVE NUMBER

// 2's compliment = 1's compliment + 1.

// 1's compliment is just the bits **flipped**.

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Check if a number is odd or even.

For checking a number n is odd or even, we generally do this :

int n;
if (n%2 == 0)
{
    cout << "n is even";
}

But, we can also do this to check if a number is odd or even :

int n;
if (n&1 == 1)
{
    cout << "n is odd";
}
else
{
    cout << "n is even";
}

(any_number & 1) == 1 only if the number is odd.

(any_number & 1) == 0 only if the number is even.

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Power of 2 alternative method

n = 8 check if it is a power of 2.

if number of set bits is equal to 1, then, the number is a power of 2.

int count = 0;
while(n != 0)
{
    if (n&1 == 1)
    {
        coount++;
    }
    n = (n>>1);
}
if (count == 1)
{
    cout << "power of 2.";
}
else
{
    cout << "not power of 2";
}

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XOR

0 ^ x = x

x ^ x = 0

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Pre-defined Functions in C++

min(), max(), sort(), swap()

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Binary Search Index Trick

In binary search, if :

start = INT_MIN

and

end = INT_MAX,

then,

mid = (start + end)/2

will be out of range of int datatype in C++. So, we should be clever and use the following relation for mid.

mid = (start/2 + end/2);

// or

mid = start + (end - start)/2

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Algorithm Complexity

Linear Search : O(n)

Binary Search : O(log n)

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Lecture-11 didn't have any programming. It talked about space and time complexity.

lecture.11 notes

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LeetCode Problems Code

LeetCode - 186. Reverse Words in a String II

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Good Artile for Pointers

GFG - Pointers

Character Arrays and Pointers

Pointers and Functions