From 4bd62623b616958316cbcb929418fe9e29480748 Mon Sep 17 00:00:00 2001
From: Sreeja-99 <75175169+Sreeja-99@users.noreply.github.com>
Date: Tue, 3 Feb 2026 21:12:46 -0600
Subject: [PATCH 1/2] Add combination sum solutions with backtracking

Implemented two methods for combination sum problem using DFS with backtracking. The first method allows repeated elements, while the second uses a for loop to iterate through candidates.
---
 Leetcode_39.java | 71 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 71 insertions(+)
 create mode 100644 Leetcode_39.java

diff --git a/Leetcode_39.java b/Leetcode_39.java
new file mode 100644
index 00000000..04bda044
--- /dev/null
+++ b/Leetcode_39.java
@@ -0,0 +1,71 @@
+//Way1
+//DFS - With backtracking
+//Choose case - Same element can be choosen again - So, pass i as i only
+//No choose case - element is not selected - So, move to next element. i.e., i+1
+//Base case - check if sum==0. If yes, add to result
+//And every step, back track the path to regain the orifinal state
+//TC: 2^(m+n)
+//SC: m+n
+class Solution {
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        List<List<Integer>> res=new ArrayList<>();
+        dfs(candidates,0,target,new ArrayList<>(),res);
+        return res;
+        
+    }
+    private void dfs(int[] candidates, int i, int target, List<Integer> path, List<List<Integer>> result){
+        //base case
+        if(target<0 || i==candidates.length){
+            return;
+        }
+
+        if(target==0){
+            result.add(new ArrayList<>(path));
+            return;
+        }
+
+
+        //actual logic
+        //no choose
+        dfs(candidates,i+1,target,path,result);
+
+        //choose
+        path.add(candidates[i]);
+        dfs(candidates,i,target-candidates[i],path,result);
+        //backtrack
+        path.remove(path.size()-1);
+    }
+}
+
+
+//way2
+//for loop based recurssion - with backtracking
+//in for loop - start with i=0
+//call with same i=0 till target becomes to negative
+//Once target = 0, add the path to result
+//And when iteration returned, new one will start with i+1
+//process goes on
+//TC: O(n * 2^(m+n)) where m is the candidates length and n is the target
+//SC: O(n * h) -> O(n^2)
+class Solution {
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        List<List<Integer>> ans = new ArrayList<>();
+        helper(candidates,0,target,new ArrayList<>(),ans);
+        return ans;
+        
+    }
+    private void helper(int[] candidates, int pivot, int target,List<Integer> path, List<List<Integer>> ans){
+        if(target<0){
+            return;
+        }
+        if(target==0){
+            ans.add(new ArrayList<>(path));
+        }
+
+        for(int i=pivot;i<candidates.length;i++){
+            path.add(candidates[i]);
+            helper(candidates,i,target - candidates[i],path,ans);
+            path.remove(path.size()-1);
+        }
+    }
+}

From 621da513d4ca215772adaef3c56d3d2802e18e78 Mon Sep 17 00:00:00 2001
From: Sreeja-99 <75175169+Sreeja-99@users.noreply.github.com>
Date: Tue, 3 Feb 2026 21:13:37 -0600
Subject: [PATCH 2/2] Implement addOperators method for target calculation

---
 Leetcode_282.java | 112 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 112 insertions(+)
 create mode 100644 Leetcode_282.java

diff --git a/Leetcode_282.java b/Leetcode_282.java
new file mode 100644
index 00000000..095638f4
--- /dev/null
+++ b/Leetcode_282.java
@@ -0,0 +1,112 @@
+//Way1
+//Generate all possible numbers
+//Apply all possible combinations
+//Once last index is reached, check whether target is made or not
+//If equals to target, add to result
+//TC: O(4ⁿ * n
+//SC: n^2
+class Solution {
+    List<String> result;
+    public List<String> addOperators(String num, int target) {
+        this.result=new ArrayList<>();
+        helper(num,0,0,0,"",target);
+        return result;
+        
+    }
+
+    private void helper(String num, int pivot, long calc, long prev, String path, int target){
+        //
+        if(pivot==num.length()){
+            if(calc==target){
+                result.add(path);
+                return;
+            }
+        }
+
+
+
+        for(int i=pivot;i<num.length();i++){
+            if(num.charAt(pivot)=='0' && i!=pivot) break;
+            long curr=Long.parseLong(num.substring(pivot,i+1));
+            if(pivot==0){
+                helper(num,i+1,curr,curr,path + curr,target);
+            }else{
+                //+
+                helper(num,i+1,calc+curr,curr,path + '+' + curr,target);
+
+
+                //-
+                helper(num,i+1,calc-curr,-curr,path + '-' + curr,target);
+
+
+                //*
+                helper(num,i+1,calc-prev+prev*curr,curr*prev,path + '*' + curr,target);
+            }
+        }
+    }
+}
+
+
+//way2
+//Same prev approach - with string builder
+//Generate all possible numbers
+//Apply all possible combinations
+//Once last index is reached, check whether target is made or not
+//If equals to target, add to result
+//TC: O(4^n)
+//SC: n
+class Solution {
+    List<String> result;
+    public List<String> addOperators(String num, int target) {
+        this.result=new ArrayList<>();
+        helper(num,0,0,0,new StringBuilder(),target);
+        return result;
+        
+    }
+
+    private void helper(String num, int pivot, long calc, long prev, StringBuilder path, int target){
+        //
+        if(pivot==num.length()){
+            if(calc==target){
+                result.add(path.toString());
+                return;
+            }
+        }
+
+
+
+        for(int i=pivot;i<num.length();i++){
+            if(num.charAt(pivot)=='0' && i!=pivot) break;
+            long curr=Long.parseLong(num.substring(pivot,i+1));
+            if(pivot==0){
+                int len=path.length();
+                path.append(curr);
+                helper(num,i+1,curr,curr,path,target);
+                path.setLength(len);
+            }else{
+                //+
+                int len=path.length();
+                path.append('+');
+                path.append(curr);
+                helper(num,i+1,calc+curr,curr,path,target);
+                path.setLength(len);
+
+
+                //-
+                 len=path.length();
+                path.append('-');
+                path.append(curr);
+                helper(num,i+1,calc-curr,-curr,path,target);
+                path.setLength(len);
+
+
+                //*
+                 len=path.length();
+                path.append('*');
+                path.append(curr);
+                helper(num,i+1,calc-prev+prev*curr,curr*prev,path,target);
+                path.setLength(len);
+            }
+        }
+    }
+}