Expression Add Operators

Recursion/GenerateExpression.java

@@ -0,0 +1,65 @@
+
+import java.util.ArrayList;
+import java.util.List;
+
+//282 leetcode with minor difference
+//TC = O( 3 ^ n-1) * n
+//SC = O(n) + O(1)*n + O( 3 ^ n-1) * n
+public class GenerateExpression {
+
+    static public List<String> result = new ArrayList<>();
+    static public long target = 24;
+
+
+    private static void helper(String nums, int idx, List<Character> slate, long target, long curSum, Long curProduct, int curNum) {
+        //backTracking
+        if(curSum > target)
+            return;
+
+        if(idx == nums.length()) {
+            if(curSum+curProduct*curNum == target) {
+                StringBuilder sb = new StringBuilder();
+                for(char i : slate) {
+                    sb.append(i);
+                }
+                result.add(sb.toString());
+            }
+            return;
+        }
+
+        //concatinate
+        slate.add(nums.charAt(idx));
+        helper(nums,idx+1, slate, target, curSum, curProduct, curNum*10+(nums.charAt(idx) -'0'));
+        slate.remove(slate.size()-1);
+
+        //"+"
+        slate.add('+');
+        slate.add(nums.charAt(idx));
+        helper(nums,idx+1, slate, target, curSum+curProduct*curNum, 1L, nums.charAt(idx)-'0');
+        slate.remove(slate.size()-1);
+        slate.remove(slate.size()-1);
+
+        //*
+        slate.add('*');
+        slate.add(nums.charAt(idx));
+        helper(nums,idx+1, slate, target, curSum, curProduct*curNum, nums.charAt(idx)-'0');
+        slate.remove(slate.size()-1);
+        slate.remove(slate.size()-1);
+
+    }
+
+
+    public static void main(String[] args) {
+        String nums = "1234";
+        List<Character> slate = new ArrayList<>();
+        slate.add(nums.charAt(0));
+        Long curSum = 0L;
+        long curProduct = 1L;
+        int idx = 0;
+        helper(nums, 1, slate, 11, 0L, 1L, nums.charAt(0)-'0');
+        System.out.println(result);
+    }
+
+}
+
+