Math

Basic

while (x) {
    int temp = ans * 10 + x % 10;
    if (temp / 10 != ans) return 0;
    ans = temp;
    x /= 10;
}

Plus One

• Input may contain leading zeros
• Need to save carry
• Check for carry after loop

Reverse Integer

• Watch out for overflow
• Can divide by 10 to check for overflow

Write your own atoi

• Carefully consider all possible input cases.
• Leading zeros
• Contain optional sing (plus or minus)
• Convert digit char to integer
• Check for overflow

Excel Sheet Column Number

• 26 base
• Need to check overflow

Excel Sheet Column Title

• There's no representation of 0
• Tricky part is how to handle 27
• We can decrement n by one before any calculation
• so 27 will be (27-1) % 26 = 0, (27-1)/26 = 1

Roman to Integer

• Take symbol one by one from starting str.size()-2
• If current value of symbol is greater than or equal to the value of next symbo, then add this value to the sum
• Else subtract this value from the sum

Happy Number

• Similar to check cycle in linked list
• Use fast pointer and slow pointer