Added solution for Find single integer

problem/find-the-single-integer_en.md

@@ -1,4 +1,76 @@
+> This awesome solution is created by [@gsriram24]([https://github.com/pinglu85](https://github.com/gsriram24)) !
 
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+In this solution, we use the concept of bitwise operators and `Array.prototype.reduce` function to get the only element that is available.
+
+### XOR Bitwise operator in Javascript
+
+In JavaScript, the bitwise XOR(^) Operator is used to compare two operands and return a new binary number which is 1 if both the bits in operators are different and 0 if both the bits in operads are the same. The operation is represented by the `^` symbol. This operator can be used to find the missing numbers in an array of natural numbers.
+
+Eg. 
+```
+     9 (base 10) = 00000000000000000000000000001001 (base 2)
+    14 (base 10) = 00000000000000000000000000001110 (base 2)
+                   --------------------------------
+14 ^ 9 (base 10) = 00000000000000000000000000000111 (base 2) = 7 (base 10)
+```
+
+Read more about bitwise XOR [here](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_XOR)
+
+We can use this to determine which of the numbers have a missing counterpart in our array using the following steps. 
+
+1. Have a result variable initialized to 0. 
+2. Iterate through each element and apply the XOR operation between the result and the element. 
+3. Return the result element. 
+
+Let's take a simple example to better understand these steps. 
+
+`arr = [2,1,2]`
+
+`result = 0`
+
+**Iteration 1**: Element: `2`. Binary representation: `10`.  `result = result ^ element => 00 ^ 10 => 10 => 2`.
+
+**Iteration 2**: Element: `1`. Binary representation: `01`.  `result = result ^ element => 10 ^ 01 => 11 => 3`. (Please stay with me here, although 3 is not present in the array, we will cancel it out. :))
+
+**Iteration 3**: Element: `2`. Binary representation: `10`.  `result = result ^ element => 11 ^ 10 => 01 => 1`.
+
+Now we return the result of 1. 
+
+The javascript implementation of this using `Array.prototype.forEach` will be as follows: 
+
+```javascript
+/**
+ * @param {number[]} arr
+ * @returns number
+ */
+function findSingle(arr) {
+  let result = 0;
+  arr.forEach(el => result = result ^ el)
+  return result;
+}
+```
+This is a very valid simple 3-liner solution to solve this problem. But we can simplify this further.
+
+### Array.prototype.reduce
+
+The Javascript `arr.reduce()` method in JavaScript is used to reduce the array to a single value and executes a provided function for each value of the array (from left to right) and the return value of the function is stored in an accumulator. 
+
+In our case, the accumulator is the `result` variable. And we would xor it with itself each iteration.
+
+Read more about `Array.prototype.reduce` [here](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce)
+
+### Final implementation
+```javascript
+/**
+ * @param {number[]} arr
+ * @returns number
+ */
+function findSingle(arr) {
+  return arr.reduce((acc, value) => acc ^= value, 0)
+}
+```
+
+### Time and space complexity
+The time complexity of this approach is `O(n)` as we would be iterating through the entire array once.
+The space complexity of this approach is `O(1)` as we don't need any additional space.