Below code snippet from Leetcode. the given exercise is to find the longest substring without repeating characters. I am trying to understand the logic from someone has posted the solution

I have below question is

1. I have cnt and s are array. is this array inside array cnt[s[j]] and cnt[s[j]] ? how it works, please help to explain. I have tried to visualize the code execution using this

I have tried to understand below line . I tried to visualize the code execution using

#include <stdio.h>

int lengthOfLongestSubstring(char * s)
{
    if (s[0] == '\0')
        return 0;
    if (s[1] == '\0')
        return 1;
    
    int i,  j, len, max = 0;
    int cnt[255] = {0}; // array of counter
    //memset(cnt,0,sizeof(cnt));
    for (i=0; s[i]!=0; i  )
    {
        len = 0;
        for (j=i; s[j]!=0; j  )
        {  
            if (cnt[s[j]] == 0)          /* What does this mean since cnt and s both are array? is this called array inside array ? */
            {
                printf("iteration %d  %c\n",j,s[j]);
                cnt[s[j]]  ;
                len  ;
            }
            else
            { /* if character are not equal */
                break;
            }
        }
        if (len > max)
            max = len;
    }
    return max;
}

int main()
{
    char string1[] = "abcabcbb";
    printf("%d",lengthOfLongestSubstring(string1));
    return 0;
}

CodePudding user response：

The syntax a[b[i]] means the value in b[i] references the index from a to read.

So if you have int a[] = { 10, 100, 1000, 10000, 100000}; int b[] = { 3, 2, 1, 0}; then a[b[0]] resolves to a[3] which has the value 10000.

Note that this requires b to only have values that are valid indexes into a.

CodePudding user response：

It's not an array inside an array, it's using one array to get the subscript into another array.

When you see a complex expression you don't understand, split it up into simpler expressions.

cnt[s[j]]  ;

is roughly equivalent to

int charcode = s[j];
cnt[charcode]  ;

s is a string, so s[j] contains a character code. So this increments the element of cnt corresponding to that character code, and the final result is frequency counts of each character.