2. Group Anagrams

Pattern: HashMap

Problem

Given an array of strings, group the anagrams together.

Example:

Input:  ["eat", "tea", "tan", "ate", "nat", "bat"]
Output: [["eat","tea","ate"], ["tan","nat"], ["bat"]]

Solution

func groupAnagrams(strs []string) [][]string {
    groups := map[[26]int][]string{}

    for _, s := range strs {
        var key [26]int
        for _, c := range s {
            key[c-'a']++
        }
        groups[key] = append(groups[key], s)
    }

    result := make([][]string, 0, len(groups))
    for _, group := range groups {
        result = append(result, group)
    }
    return result
}

Key insight: [26]int (fixed-size array) can be used as map key in Go, but []int (slice) cannot. Arrays are value types and compared element by element.

1. Two Sum 3. Longest Consecutive Sequence