Create-lc-0017-Letter-Combinations-of-a-Phone-Number

katarianikita2003 · katarianikita2003 · commit 3ec8a45cb3c8 · 2024-06-09T14:21:14.000+05:30
Created leetcode solution on problem 0017 using python, java, typescript, javascript, cpp
diff --git a/dsa-solutions/lc-solutions/0000-0099/0017-Letter-Combinations-of-a-Phone-Number.md b/dsa-solutions/lc-solutions/0000-0099/0017-Letter-Combinations-of-a-Phone-Number.md
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+---
+id: Letter Combinations of a Phone Number
+title: Letter Combinations of a Phone Number (LeetCode)
+sidebar_label: 0017-Letter-Combinations-of-a-Phone-Number
+tags:
+    - Back Tracking
+    - Mapping
+    - String
+description: The problem requires generating all letter combinations corresponding to given digits (2-9). The solution utilizes backtracking to explore all combinations efficiently, employing a recursive approach in Java.
+---
+
+## Problem Description
+
+| Problem Statement                                                                                           | Solution Link                                                                                                                               | LeetCode Profile                                   |
+| :----------------------------------------------------------------------------------------------------------- | :------------------------------------------------------------------------------------------------------------------------------------------ | :------------------------------------------------- |
+| [Letter Combinations of a Phone Number](https://leetcode.com/problems/Letter Combinations of a Phone Number/)                                         | [Letter Combinations of a Phone Number Solution on LeetCode](https://leetcode.com/problems/Letter Combinations of a Phone Number/solutions/5055810/video-two-pointer-solution/) | [gabaniyash846](https://leetcode.com/u/gabaniyash846/) |
+
+### Problem Description
+
+## Problem Statement:
+Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.
+
+### Examples
+
+#### Example 1
+
+- **Input:** `digits = "23"`
+- **Output:** `["ad","ae","af","bd","be","bf","cd","ce","cf"]`
+
+#### Example 2
+
+- **Input:** `digits = ""`
+- **Output:** `[]`
+
+
+#### Example 3
+
+- **Input:** `2`
+- **Output:** `["a","b","c"]`
+
+### Constraints:
+- `0 ≤ digits.length ≤ 4`
+- `0 ≤ digits.length ≤ 4digits[𝑖]`
+- `digits[i] is a digit in the range ['2', '9'].`
+- `A mapping of digits to letters (similar to telephone buttons) is given below. Note that 1 does not map to any letters.`
+
+### Approach
+
+1. **Mapping Digits to Letters:**
+   - Define a mapping of digits to their corresponding letters, similar to telephone buttons.
+
+2. **Backtracking Function:**
+   - Define a recursive backtracking function to generate all possible combinations.
+   - The function takes four parameters:
+     - `index`: The current index in the digits string.
+     - `path`: The current combination of letters.
+   - If the index is equal to the length of the digits string, it means we have reached the end of a combination, so we add it to the result list.
+   - Otherwise, for each letter corresponding to the current digit, we append it to the current combination and recursively call the function with the next index.
+   - After the recursive call, we remove the last character from the combination (backtracking).
+
+3. **Base Case:**
+   - If the length of the current combination is equal to the length of the input digits string, we add the combination to the result list.
+
+4. **Main Function:**
+   - Initialize an empty list to store the combinations.
+   - Call the backtracking function with the initial index set to 0 and an empty string as the initial combination.
+   - Return the list of combinations.
+
+This approach ensures that all possible combinations are generated using backtracking, and the result is returned in the desired format.
+
+### Solution Code
+
+#### Python
+
+```python
+class Solution:
+    def letterCombinations(self, digits: str) -> List[str]:
+        if not digits:
+            return []
+
+        digit_to_letters = {
+            '2': 'abc',
+            '3': 'def',
+            '4': 'ghi',
+            '5': 'jkl',
+            '6': 'mno',
+            '7': 'pqrs',
+            '8': 'tuv',
+            '9': 'wxyz'
+        }
+
+        def backtrack(index, path):
+            if index == len(digits):
+                combinations.append(path)
+                return
+            for letter in digit_to_letters[digits[index]]:
+                backtrack(index + 1, path + letter)
+
+        combinations = []
+        backtrack(0, '')
+        return combinations
+```
+
+#### Java
+
+```java
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+public class Solution {
+    private Map<Character, String> digitToLetters = new HashMap<>();
+
+    public Solution() {
+        digitToLetters.put('2', "abc");
+        digitToLetters.put('3', "def");
+        digitToLetters.put('4', "ghi");
+        digitToLetters.put('5', "jkl");
+        digitToLetters.put('6', "mno");
+        digitToLetters.put('7', "pqrs");
+        digitToLetters.put('8', "tuv");
+        digitToLetters.put('9', "wxyz");
+    }
+
+    public List<String> letterCombinations(String digits) {
+        List<String> combinations = new ArrayList<>();
+        if (digits == null || digits.isEmpty()) {
+            return combinations;
+        }
+        backtrack(combinations, digits, 0, new StringBuilder());
+        return combinations;
+    }
+
+    private void backtrack(List<String> combinations, String digits, int index, StringBuilder path) {
+        if (index == digits.length()) {
+            combinations.add(path.toString());
+            return;
+        }
+        String letters = digitToLetters.get(digits.charAt(index));
+        for (char letter : letters.toCharArray()) {
+            path.append(letter);
+            backtrack(combinations, digits, index + 1, path);
+            path.deleteCharAt(path.length() - 1);
+        }
+    }
+
+    public static void main(String[] args) {
+        Solution solution = new Solution();
+        List<String> result = solution.letterCombinations("23");
+        System.out.println(result); // Output: [ad, ae, af, bd, be, bf, cd, ce, cf]
+    }
+}
+```
+
+#### CPP:
+```cpp
+#include <iostream>
+#include <vector>
+#include <unordered_map>
+
+using namespace std;
+
+class Solution {
+private:
+    unordered_map<char, string> digitToLetters;
+    vector<string> combinations;
+
+public:
+    Solution() {
+        digitToLetters = {
+            {'2', "abc"},
+            {'3', "def"},
+            {'4', "ghi"},
+            {'5', "jkl"},
+            {'6', "mno"},
+            {'7', "pqrs"},
+            {'8', "tuv"},
+            {'9', "wxyz"}
+        };
+    }
+
+    vector<string> letterCombinations(string digits) {
+        if (digits.empty()) return {};
+        backtrack(digits, 0, "");
+        return combinations;
+    }
+
+    void backtrack(const string& digits, int index, string path) {
+        if (index == digits.length()) {
+            combinations.push_back(path);
+            return;
+        }
+        for (char letter : digitToLetters[digits[index]]) {
+            backtrack(digits, index + 1, path + letter);
+        }
+    }
+};
+
+int main() {
+    Solution solution;
+    vector<string> result = solution.letterCombinations("23");
+    for (const string& comb : result) {
+        cout << comb << " ";
+    }
+    // Output: ad ae af bd be bf cd ce cf
+    return 0;
+}
+```
+
+#### JavaScript
+```js
+/**
+ * @param {string} digits
+ * @return {string[]}
+ */
+var letterCombinations = function(digits) {
+    if (digits.length === 0) return [];
+    
+    const digitToLetters = {
+        '2': 'abc',
+        '3': 'def',
+        '4': 'ghi',
+        '5': 'jkl',
+        '6': 'mno',
+        '7': 'pqrs',
+        '8': 'tuv',
+        '9': 'wxyz'
+    };
+    
+    const combinations = [];
+    
+    const backtrack = (index, path) => {
+        if (index === digits.length) {
+            combinations.push(path);
+            return;
+        }
+        const letters = digitToLetters[digits.charAt(index)];
+        for (let letter of letters) {
+            backtrack(index + 1, path + letter);
+        }
+    };
+    
+    backtrack(0, '');
+    return combinations;
+};
+
+// Example usage:
+console.log(letterCombinations("23")); // Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
+```
+
+#### TypeScript
+```ts
+class Solution {
+    private digitToLetters: { [key: string]: string } = {
+        '2': 'abc',
+        '3': 'def',
+        '4': 'ghi',
+        '5': 'jkl',
+        '6': 'mno',
+        '7': 'pqrs',
+        '8': 'tuv',
+        '9': 'wxyz'
+    };
+
+    letterCombinations(digits: string): string[] {
+        const combinations: string[] = [];
+
+        const backtrack = (index: number, path: string): void => {
+            if (index === digits.length) {
+                combinations.push(path);
+                return;
+            }
+            const letters = this.digitToLetters[digits.charAt(index)];
+            for (let letter of letters) {
+                backtrack(index + 1, path + letter);
+            }
+        };
+
+        if (digits.length !== 0) {
+            backtrack(0, '');
+        }
+        
+        return combinations;
+    }
+}
+
+// Example usage:
+const solution = new Solution();
+console.log(solution.letterCombinations("23")); // Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
+```
+
+### Step-by-Step Algorithm
+
+Here's a step-by-step algorithm for generating all possible letter combinations of a given string of digits using backtracking:
+
+1. **Define a mapping of digits to letters:**
+   - Create a map where each digit from 2 to 9 is mapped to its corresponding letters on a telephone keypad.
+
+2. **Define a backtracking function:**
+   - The function will take the following parameters:
+     - `index`: The current index in the digits string.
+     - `path`: The current combination of letters.
+   - If the index is equal to the length of the digits string, it means we have formed a complete combination, so add it to the result list.
+   - Otherwise, for each letter corresponding to the current digit at the given index, append it to the current combination and recursively call the function with the next index.
+   - After the recursive call, remove the last character from the combination (backtracking).
+
+3. **Base Case:**
+   - If the length of the current combination is equal to the length of the input digits string, add the combination to the result list.
+
+4. **Main Function:**
+   - Initialize an empty list to store the combinations.
+   - Call the backtracking function with the initial index set to 0 and an empty string as the initial combination.
+   - Return the list of combinations.
+
+This algorithm ensures that all possible combinations are generated by exploring all valid paths through backtracking.
