Brute Force Approach

Algorithm

Take two pointers let’s say i,j pointing to the 0th index of S1 and S2.
Traverse the two strings until we reach the end of any string.
In the iteration compare the S1[ i ] and S2[ j ], if S1[ i ] == S2[ j ] that mean we found the character corresponding to S1[ i ] in S2 ,So then increment both i and j.
If S1[ i ] != S2[ j ] that mean we not found the character corresponding to S1 [ i ] in S2.
So increment only j.
After the traversing check whether the ( i == S1.length) if i is equal to S1’s length, It means, we found every character of S1 in S2 without distributing the relative order of S2. Hence return true.
If (i != S1.length) there are still characters left in S1 and S2 if finished, So S1 cannot be the Subsequence of S2.

Code