Merge pull request neetcode-gh#373 from evaunit00/main

neetcode-gh · web-flow · commit aa439376ab4f · 2022-07-06T15:38:42.000-07:00
Added question 17 and 39 in Java
diff --git a/java/17-Letter-Combinations-of-a-Phone-Number.java b/java/17-Letter-Combinations-of-a-Phone-Number.java
@@ -0,0 +1,37 @@
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+
+class Solution {
+    private Map<Character, String> digitToChar = Map.of(
+        '2', "abc", '3', "def", '4', "ghi", '5', "jkl",
+        '6', "mno", '7', "pqrs", '8', "tuv", '9', "wxyz");
+    
+    public List<String> letterCombinations(String digits) {
+     
+        if (digits.length() == 0) {
+            return new ArrayList();
+        }
+        
+        List<String> ans = new ArrayList();
+        String cur = "";
+        backtrack(digits, ans, cur, 0);
+        return ans;
+        
+    }
+    
+    public void backtrack(String digits, List<String> ans, String cur, int index) {
+        
+        if (cur.length() == digits.length()) {
+            ans.add(cur);
+            return;
+        } else if (index >= digits.length()) {
+            return;
+        } else {
+            String digit = digitToChar.get(digits.charAt(index));
+            for (char c : digit.toCharArray()) {
+                backtrack(digits, ans, cur + c, index + 1);
+            }
+        }
+    }
+}
diff --git a/java/39-Combination-Sum.java b/java/39-Combination-Sum.java
@@ -0,0 +1,24 @@
+class Solution {
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        List<List<Integer>> ans = new ArrayList<List<Integer>>();
+        List<Integer> cur = new ArrayList();
+        backtrack(candidates, target, ans, cur, 0);
+        return ans;
+        
+    }
+    
+    public void backtrack(int[] candidates, int target, List<List<Integer>> ans, List<Integer> cur, int index) {
+        if (target == 0) {
+            ans.add(new ArrayList(cur));
+        } else if (target < 0 || index >= candidates.length) {
+            return;
+        } else {
+            cur.add(candidates[index]);
+            backtrack(candidates, target - candidates[index], ans, cur, index);
+            
+            cur.remove(cur.get(cur.size() - 1));
+            backtrack(candidates, target, ans, cur, index + 1);
+        }
+        
+    }    
+}
