Check if a binary string has a 0 between 1s or not | Set 1 (General approach)

Given a string of 0 and 1, we need to check that the given string is valid or not. The given string is valid when there is no zero present in between 1's. For example, 1111, 0000111110, 1111000 are valid strings but 01010011, 01010, 101 are not. 

One approach to solving the problem is discussed here, other using Regular expressions is given Set 2 

Examples: 

Input : 100
Output : VALID

Input : 1110001
Output : NOT VALID
There is 0 between 1s

Input : 00011
Output : VALID

Approach 1 : (Simple Traversal)

Suppose we have a string: 01111011110000 Now take two variables say A and B. run a loop for 0 to length of the string and point A to the first occurrence of 1, after that again run a loop from length of the string to 0and point second variable to the last occurrence of 1.So A = 1 (Position of first '1' is the string) and B = 9 (last occurrence of '1').Now run a loop from A to B and check that '0' is present between 1 or not, if "YES" than set flag to 1 and break the loop, else set flag to 0.In this case flag will set to 1 as the given string is not valid and print "NOT VALID".

Implementation:

VALID

Time Complexity: O(n)

Auxiliary Space: O(1) since it is using constant space for variables

Approach 2 : (Using Two Pointers)

Will keep two pointers on the both end of the string and will check whether in the both side '1' is found (track the index with another variable) or not. If in the both sides we find '1' then will run another loop for the inner part using the other two variables, which will again be a part of two pointers algorithm.

Look at the code for better understanding,

VALID

Time Complexity : O(N)
Auxiliary Space : O(1), since it is using constant space for variables.