Implement an space efficient algorithm to determine if a string (of characters from ‘a’ to ‘z’) has all unique characters or not. Use additional data structures like count array, hash, etc is not allowed.

Expected Time Complexity : O(n)

Input : str = "aaabbccdaa" Output : No Input : str = "abcd" Output : Yes

The idea is to use an integer variable and use bits in its binary representation to store whether a character is present or not. Typically an integer has at-least 32 bits and we need to store presence/absence of only 26 characters.

Below is C++ implementation of the idea.

// A space efficient C++ program to check if // all characters of string are unique. #include<bits/stdc++.h> using namespace std; // Returns true if all characters of str are // unique. // Assumptions : (1) str contains only characters // from 'a' to 'z' // (2) integers are stored using 32 // bits bool areChractersUnique(string str) { // An integer to store presence/absence // of 26 characters using its 32 bits. int checker = 0; for (int i = 0; i < str.length(); ++i) { int val = (str[i]-'a'); // If bit corresponding to current // character is already set if ((checker & (1 << val)) > 0) return false; // set bit in checker checker |= (1 << val); } return true; } // Driver code int main() { string s = "aaabbccdaa"; if (areChractersUnique(s)) cout << "Yes"; else cout << "No"; return 0; }

Output:

No

Time Complexity: O(n)

Auxiliary Space : O(1)

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