Can someone help me to understand below syntax... thanks in advance.

class Solution {
public:
int lengthOfLongestSubstring(string s) {
    const int n = s.length(); 
    int ans = 0; //set a var as answer;
        for(int i = 0; i < n;   i){
            vector <int> seen(128); 
            int j = i;
                while(j < n && !seen[s[j]]) <----here
                seen[s[j  ]]=1;             <----here
                ans=max(ans, j - i);        <----here
  }
    return ans;
 }
};
                

wasn't sure what the syntax means !seen[s[j]]
same as this seen[s[j ]]=1 also don't know why i can be subtracted by j...

CodePudding user response：

Case 1

s[j]

The above means the element at index j of the string named s.

Case 2

seen[s[j]]

The above means the element at index s[j] of variable named seen.

Case 3

seen[s[j  ]]=1;

For the above you have to know about the post-increment operator. So lets say we have:

int var = 0;
std::cout << var   <<std::end; //this will print 0

The in var means we are incrementing the value of var by 1 so that it now becomes 1, but var returns the old value of the variable var which is why we get 0 as the output.

Now lets come back to:

seen[s[j  ]]=1;

The above means the element at index s[j ] of the variable named seen. But note j will do 2 things: First it will increment j by 1 and second it will return the old value of j. So s[j ] essentially means the element at index j of the string named s meanwhile the value of j is also incremented by 1. Thus as a whole, you're assigning a value of 1 to the element at index s[j] of the variable named seen meanwhile also increment the value of j by one.

why i can be subtracted by j

This is because the value of j is incremented by 1 inside the while loop.

CodePudding user response：

s[j] is the character at index j in the std::string s. seen[s[j]] is the element at index s[j] in the std::vector<int> seen.

If seen[s[j]] is 0, seen[s[j]] will return false, so !seen[s[j]] will return true. If seen[s[j]] is a nonzero value, seen[s[j]] will return true, so !seen[s[j]] will return false.

seen[s[j ]]=1; set the value of seen[s[j]] to 1, then increment j

ans = std::max(ans, j - i);, std::max(ans, j - i) will return the bigger value between ans, and j - i, then assign that value to ans