Longest Substring with At Most K Distinct Characters in C++

Suppose we have a string; we have to calculate the length of the longest substring T that contains at most k distinct characters.

So, if the input is like s = "eceba", k = 2, then the output will be 3 as T is "ece" which its length is 3.

To solve this, we will follow these steps −

• ans := 0

• Define one map m

• n := size of s

• x := 0

• for initialize j := 0, i := 0, when j < n, update (increase j by 1), do −

  • (increase m[s[j]] by 1)

  • if m[s[j]] is same as 1, then −

    • (increase x by 1)

  • while (x > k and i <= j), do −

    • (decrease m[s[i]] by 1)

    • if m[s[i]] is same as 0, then −

      • (decrease x by 1)

    • (increase i by 1)

  • ans := maximum of ans and (j - i + 1)

• return ans

Example 

Let us see the following implementation to get better understanding −

 Live Demo

#include <bits/stdc++.h>
using namespace std;
class Solution {
public:
   int lengthOfLongestSubstringKDistinct(string s, int k) {
      int ans = 0;
      unordered_map<char, int> m;
      int n = s.size();
      int x = 0;
      for (int j = 0, i = 0; j < n; j++) {
         m[s[j]]++;
         if (m[s[j]] == 1)
            x++;
         while (x > k && i <= j) {
            m[s[i]]--;
            if (m[s[i]] == 0)
               x--;
            i++;
         }
         ans = max(ans, j - i + 1);
      }
      return ans;
   }
};
main() {
   Solution ob;
   cout << (ob.lengthOfLongestSubstringKDistinct("eceba", 2));
}

Input

"eceba", 2

Output

3