C#

int len=s.Length;
int i=0;
while(i<len) {
    char c=s[i++];
    if (c=='6') {
        while(i<len) {
            c=s[i++];
            if (c=='2') {
                while(i<len) {    // FIXED
                    c=s[i++];
                    if (c=='4') return true;
                }
            }
        }
    }
}
return false;

string search="624x";  // FIXED
int iSearch=0;
char cSearch=search[iSearch];
foreach(char c in s) {
    if(c==cSearch) {
        cSearch=search[++iSearch];    // FIXED
        if (cSearch=='x') return true;
    }
}
return false;

Luc Pattyn wrote:

BTW: none if these fulfill the requirement, as they would accept 6424 whereas the OP wants to reject that; so the state machine needs to be a tiny bit more complex.

int i2=s.IndexOf('2');
if (i2>=0 && i2<s.LastIndexOf('4')) return true;

Luc Pattyn wrote:

an IndexOf-based approach, when using several calls to such methods, has the disadvantage of scanning the string several times;

Luc Pattyn wrote:

I'm not sure what you mean by my first and second approach;

Luc Pattyn wrote:

When in doubt, just give it a try.

Bassam Abdul-Baki wrote:

but I would think Microsoft has it optimized

Luc Pattyn wrote:

IndexOf is fine for short strings; it is also fine for long strings if you know the character is present. It is a waste if you need several of them and most of those return nothing.

// You'd need to add logic to test if any of the characters don't exist (i.e., if IndexOf returns -1).
int index = str.IndexOf(char3, str.IndexOf(char2, str.IndexOf(char1) + 1) + 1);

aspdotnetdev wrote:

You do know there are overloads for IndexOf ...

int pos1=s.IndexOf(char1);
int pos2=s.IndexOf(char2);
int pos3=s.IndexOf(char3);
.. and then a lot of decisions based on the value of pos1/2/3.

Luc Pattyn wrote:

And that could be very bad on long strings.

aspdotnetdev wrote:

IndexOf (using the start index) and a for loop is probably the best way to go, as Piebald suggested.

Luc Pattyn wrote:

It might be improved by looking for the last subitem using LastIndexOf

Luc Pattyn wrote:

So, yes PIEBALD's approach is a correct one and possibly the best

Luc Pattyn wrote:

cSearch=search[iSearch++];