Added some important Backtracking question.

Medium/CombinationSum.java

@@ -0,0 +1,67 @@
+<<<<<<< HEAD
+//                https://leetcode.com/problems/combination-sum/
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+public class CombinationSum {
+    public static void main(String[] args) {
+        int[] candidates = {2,3,6,7};
+        List<List<Integer>> sum = combinationSum(candidates, 7);
+        System.out.println(sum);
+    }
+    public static List<List<Integer>> combinationSum(int[] candidates, int target) {
+        List<List<Integer>> ans = new ArrayList<>();
+        findCombination(0, candidates, target, ans, new ArrayList<>());
+        return ans;
+    }
+    public static void findCombination(int index, int[] arr, int target, List<List<Integer>> ans, List<Integer> ds){
+        if(index == arr.length){
+            if(target == 0){
+                ans.add(new ArrayList<>(ds));
+            }
+            return;
+        }
+
+        if(arr[index] <= target){
+            ds.add(arr[index]);
+            findCombination(index, arr, target - arr[index], ans, ds);
+            ds.remove(ds.size()-1);
+        }
+        findCombination(index+1, arr, target, ans, ds);
+    }
+}
+=======
+//                https://leetcode.com/problems/combination-sum/
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+public class CombinationSum {
+    public static void main(String[] args) {
+        int[] candidates = {2,3,6,7};
+        List<List<Integer>> sum = combinationSum(candidates, 7);
+        System.out.println(sum);
+    }
+    public static List<List<Integer>> combinationSum(int[] candidates, int target) {
+        List<List<Integer>> ans = new ArrayList<>();
+        findCombination(0, candidates, target, ans, new ArrayList<>());
+        return ans;
+    }
+    public static void findCombination(int index, int[] arr, int target, List<List<Integer>> ans, List<Integer> ds){
+        if(index == arr.length){
+            if(target == 0){
+                ans.add(new ArrayList<>(ds));
+            }
+            return;
+        }
+
+        if(arr[index] <= target){
+            ds.add(arr[index]);
+            findCombination(index, arr, target - arr[index], ans, ds);
+            ds.remove(ds.size()-1);
+        }
+        findCombination(index+1, arr, target, ans, ds);
+    }
+}
+>>>>>>> 67aafb10a70e470ee512334062930bef20542fa0

Medium/TargetSum.java

@@ -0,0 +1,25 @@
+//  https://leetcode.com/problems/target-sum/
+public class TargetSum {
+    static int count;
+    public static void main(String[] args) {
+        int[] nums = {1, 1, 1, 1, 1};
+        System.out.println(findTargetSumWays(nums, 3));
+    }
+    public static int findTargetSumWays(int[] nums, int target){
+        count = 0;
+        TargetSum(nums, target, 0, 0);
+
+        return count;
+    }
+    public static void TargetSum(int[] nums, int target, int index, int val){
+        if(index == nums.length){
+            if(target == val){
+                count++;
+            }
+            return;
+        }
+
+        TargetSum(nums, target, index+1, val+nums[index]);
+        TargetSum(nums, target, index+1, val-nums[index]);
+    }
+}

Medium/WordSearch.java

@@ -0,0 +1,70 @@
+// https://leetcode.com/problems/word-search/
+
+public class WordSearch {
+    public static void main(String[] args) {
+        char[][] board = {
+                {'A','B','C','E'},
+                {'S','F','C','S'},
+                {'A','D','E','E'}
+        };
+        boolean ans = exist(board, "ABCCED");
+        System.out.println(ans);
+    }
+    public static boolean exist(char[][] board, String word) {
+
+        for(int i = 0; i < board.length; i++){
+            for(int j = 0; j < board[0].length; j++){
+                if(board[i][j] == word.charAt(0)){
+                    char temp = board[i][j];
+                    board[i][j] = '0';
+                    if(isValid(board, i, j, 1, word)){
+                        return true;
+                    }
+                    board[i][j] = temp;
+                }
+            }
+        }
+
+        return false;
+    }
+    public static boolean isValid(char[][] board, int row, int col, int index, String word){
+        if(index == word.length()){
+            return true;
+        }
+        char temp;
+        if(col != 0 && board[row][col - 1] == word.charAt(index)){
+            temp = board[row][col - 1];
+            board[row][col - 1] = '0';
+            if(isValid(board, row, col-1, index+1, word)){
+                return true;
+            }
+            board[row][col-1] = temp;
+        }
+        if(col <= board[0].length-2 && board[row][col + 1] == word.charAt(index)){
+            temp = board[row][col + 1];
+            board[row][col + 1] = '0';
+            if(isValid(board, row, col+1, index+1, word)){
+                return true;
+            }
+            board[row][col+1] = temp;
+        }
+        if(row != 0 && board[row - 1][col] == word.charAt(index)){
+            temp = board[row - 1][col];
+            board[row - 1][col] = '0';
+            if(isValid(board, row - 1, col, index+1, word)){
+                return true;
+            }
+            board[row - 1][col] = temp;
+        }
+        if(row <= board.length - 2 && board[row + 1][col] == word.charAt(index)){
+            temp = board[row + 1][col];
+            board[row + 1][col] = '0';
+            if(isValid(board, row + 1, col, index+1, word)){
+                return true;
+            }
+            board[row + 1][col] = temp;
+        }
+
+        return false;
+    }
+}