LeetCode 1079: Letter Tile Possibilities Solution

Master LeetCode problem 1079 (Letter Tile Possibilities), a medium challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

1079. Letter Tile Possibilities

Medium

Hash Table String Backtracking Counting

Problem Explanation

Explanation

To solve this problem, we can use backtracking to generate all possible permutations of the given tiles and count how many distinct sequences can be formed. We will keep track of the frequencies of each letter in the input tiles to avoid duplicate sequences.

Create a frequency map for the letters in the input tiles.
Use backtracking to generate all possible permutations of the tiles.
During backtracking, for each letter that has a non-zero frequency in the frequency map, we add it to the current sequence, decrement its frequency, and recursively call the backtracking function.
When the current sequence is of the same length as the input tiles, we increment the count of distinct sequences.
Backtrack by incrementing the frequency of the last added letter to the frequency map.
Return the count of distinct sequences.

Time Complexity: O(N!), where N is the number of distinct letters in the input tiles.
Space Complexity: O(N) for the frequency map and recursion stack.

Solution Code

class Solution {
    public int numTilePossibilities(String tiles) {
        int[] freq = new int[26];
        for (char c : tiles.toCharArray()) {
            freq[c - 'A']++;
        }
        return backtrack(freq);
    }
    
    private int backtrack(int[] freq) {
        int sum = 0;
        for (int i = 0; i < 26; i++) {
            if (freq[i] > 0) {
                sum++;
                freq[i]--;
                sum += backtrack(freq);
                freq[i]++;
            }
        }
        return sum;
    }
}

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