backtracking-1 done

CombinationSum.java

@@ -0,0 +1,33 @@
+// Time Complexity : O(2^(N))
+// Space Complexity :O(n)
+// Did this code successfully run on Leetcode : yes
+// Any problem you faced while coding this : no
+class Solution {
+    List<List<Integer>> res;
+
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        this.res = new ArrayList<>();
+        // Arrays.sort(candidates);
+        helper(candidates, target, 0, new ArrayList<>());
+        return res;
+    }
+
+    private void helper(int[] candidates, int target, int pivot, List<Integer> path) {
+
+        if (target == 0) {
+            res.add(new ArrayList<>(path));
+            return;
+        }
+
+        for (int i = pivot; i < candidates.length; i++) {
+            if (target - candidates[i] < 0) {
+                continue;
+            }
+            path.add(candidates[i]);
+            if (target - candidates[i] >= 0)
+                helper(candidates, target - candidates[i], i, path);
+            path.remove(path.size() - 1);
+        }
+
+    }
+}

ExpressoionAddOp.java

@@ -0,0 +1,54 @@
+// Time Complexity : O(4^(N))
+// Space Complexity :O(length of string)
+// Did this code successfully run on Leetcode : yes
+// Any problem you faced while coding this : no
+
+class Solution {
+    List<String> result;
+
+    public List<String> addOperators(String num, int target) {
+        this.result = new ArrayList<>();
+        helper(num, 0, new StringBuilder(), 0, 0, target);
+        return result;
+    }
+
+    private void helper(String num, int pivot, StringBuilder path, long calc, long tail, int target) {
+        // base
+        if (pivot == num.length()) {
+            if (target == calc) {
+                result.add(path.toString());
+            }
+        }
+
+        for (int i = pivot; i < num.length(); i++) {
+            if (num.charAt(pivot) == '0' && pivot != i)
+                continue;
+
+            long curr = Long.parseLong(num.substring(pivot, i + 1));
+
+            if (pivot == 0) {
+                int inl = path.length();
+                path.append(curr);
+                helper(num, i + 1, path, curr, curr, target);
+                path.setLength(inl);
+            } else {
+                int inl = path.length();
+
+                path.append("+");
+                path.append(curr);
+                helper(num, i + 1, path, calc + curr, curr, target);
+                path.setLength(inl);
+
+                path.append("-");
+                path.append(curr);
+                helper(num, i + 1, path, calc - curr, -curr, target);
+                path.setLength(inl);
+
+                path.append("*");
+                path.append(curr);
+                helper(num, i + 1, path, calc - tail + tail * curr, tail * curr, target);
+                path.setLength(inl);
+            }
+        }
+    }
+}