New java solution to problem 1946 and updated readme (haoel#268)

* Create Java solution for problem 1946

ran fine on leetcode, but was having some potential issues with unwanted non printable ascii characters

* update readme for java solution on 1946

Author: willmccarten

README.md

@@ -9,7 +9,7 @@ LeetCode
 
 | # | Title | Solution | Difficulty |
 |---| ----- | -------- | ---------- |
-|1946|[Largest Number After Mutating Substring](https://leetcode.com/problems/largest-number-after-mutating-substring/) | [C++](./algorithms/cpp/largestNumberAfterMutatingSubstring/LargestNumberAfterMutatingSubstring.cpp)|Medium|
+|1946|[Largest Number After Mutating Substring](https://leetcode.com/problems/largest-number-after-mutating-substring/) | [C++](./algorithms/cpp/largestNumberAfterMutatingSubstring/LargestNumberAfterMutatingSubstring.cpp), [Java](./algorithms/java/src/LargestNumberAfterMutatingSubtring/largestNumberAfterMutatingSubstring.java)|Medium|
 |1945|[Sum of Digits of String After Convert](https://leetcode.com/problems/sum-of-digits-of-string-after-convert/) | [C++](./algorithms/cpp/leetcode/sumOfDigitsOfStringAfterConvert/SumOfDigitsOfStringAfterConvert.cpp)|Easy|
 |1935|[Maximum Number of Words You Can Type](https://leetcode.com/problems/maximum-number-of-words-you-can-type/) | [C++](./algorithms/cpp/maximumNumberOfWordsYouCanType/MaximumNumberOfWordsYouCanType.cpp)|Easy|
 |1884|[Egg Drop With 2 Eggs and N Floors](https://leetcode.com/problems/egg-drop-with-2-eggs-and-n-floors/) | [C++](./algorithms/cpp/eggDropWith2EggsAndNFloors/EggDropWith2EggsAndNFloors.cpp)|Medium|

algorithms/java/src/LargestNumberAfterMutatingSubstring/largestNumberAfterMutatingSubstring.java

@@ -0,0 +1,81 @@
+/***************************************************************************************************** 
+ *
+ * You are given a string num, which represents a large integer. You are also given a 0-indexed 
+ * integer array change of length 10 that maps each digit 0-9 to another digit. More formally, digit d 
+ * maps to digit change[d].
+ * 
+ * You may choose to mutate a single substring of num. To mutate a substring, replace each digit 
+ * num[i] with the digit it maps to in change (i.e. replace num[i] with change[num[i]]).
+ * 
+ * Return a string representing the largest possible integer after mutating (or choosing not to) a 
+ * single substring of num.
+ * 
+ * A substring is a contiguous sequence of characters within the string.
+ * 
+ * Example 1:
+ * 
+ * Input: num = "132", change = [9,8,5,0,3,6,4,2,6,8]
+ * Output: "832"
+ * Explanation: Replace the substring "1":
+ * - 1 maps to change[1] = 8.
+ * Thus, "132" becomes "832".
+ * "832" is the largest number that can be created, so return it.
+ * 
+ * Example 2:
+ * 
+ * Input: num = "021", change = [9,4,3,5,7,2,1,9,0,6]
+ * Output: "934"
+ * Explanation: Replace the substring "021":
+ * - 0 maps to change[0] = 9.
+ * - 2 maps to change[2] = 3.
+ * - 1 maps to change[1] = 4.
+ * Thus, "021" becomes "934".
+ * "934" is the largest number that can be created, so return it.
+ * 
+ * Example 3:
+ * 
+ * Input: num = "5", change = [1,4,7,5,3,2,5,6,9,4]
+ * Output: "5"
+ * Explanation: "5" is already the largest number that can be created, so return it.
+ * 
+ * Constraints:
+ * 
+ * 	1 <= num.length <= 10^5
+ * 	num consists of only digits 0-9.
+ * 	change.length == 10
+ * 	0 <= change[d] <= 9
+ ******************************************************************************************************/
+
+class Solution {
+    public String maximumNumber(String num, int[] change) {
+        
+        //make flag for checking replacement
+        boolean shouldReplace = false;
+        
+        //work through substring
+        for(int i = 0; i < num.length(); i++){
+            
+            //subtract zero to get correct ascii conversion
+            char a = num.charAt(i);
+            char curr = (char)(a - '0');
+            
+            if(curr < change[curr]){
+                
+                char c = (char)(change[curr] + '0');
+                
+                num = num.replace(num.charAt(i), c);
+                
+                shouldReplace = true;
+                
+            }else if((curr > change[curr]) && (shouldReplace)){
+                
+                break;
+                
+            } 
+        }
+        
+        return num;
+    }
+}
+
+