Valid Anagram

Problem Statement

Given two strings s and t, return true if t is an anagram of s, and false otherwise.

An Anagram is a word or phrase formed by rearranging the letters of a different word or phrase, typically using all the original letters exactly once.

Note: s and t consist of lowercase English letters

Example: s = "anagram" , t = "nagaram"

returns true as the letters of anagram can be rearranged to form nagaram

Solutions

Edge case: When String lengths unequal $\implies$ NOT anagrams

1. Track Character counts

• You can use a HashMap to count occurence of each character. The character counts of both must be equal.
• Since, we only have lowercase alphabets in string, we can get away with integer arrays of length 26
• We don’t need to maintain two arrays/maps. We can just increment counts while first string’s traversal and decrement them in second string’s traversal This also prevents overflow if high character counts.

bool isAnagram(string str1, string str2) {
    int charCounts[26];             // <- fixed space

    for (char &c : str1) {          // <- O(N)
        charCounts[c - 'a']++;
    }
    for (char &c : str2) {          // <- O(N)
        charCounts[c - 'a']--;
    }
    for (int &cnt : charCounts) {   // <- O(N)
        if (cnt != 0) {
            return false;
        }
    }
    return true;
}

Time $= 3 \cdot O(N) = O(N)$

2. Sorting

When sorted, both the strings must be the same

sort(str1);                 // O(NlogN)
sort(str2);                 // O(NlogN)
return (str1 == str2);      // O(N) for comparing character-by-character

Time $= 2 \cdot O(N \cdot logN ) + O(N) = O(N \cdot logN )$

The complexity depends on the underlying sorting method used. We have assumed a good sorting time of O(NlogN) and no extra space

Variants

• If the input has UPPERCASE characters, we need to convert to lowercase and then check counts
• If the input has whitespaces too, ignore them. In such cases, the lengths of strings can be different even if they ARE anagrams