solves find the middle index in array

Author: anishLearnsToCode

Diff for: README.md

@@ -476,7 +476,7 @@
 | 1974 | [Minimum Time to Type Word Using Special Typewriter](https://leetcode.com/problems/minimum-time-to-type-word-using-special-typewriter)                    |                                                   [![Java](assets/java.png)](src/MinimumTimeToTypeWordUsingSpecialTypewriter.java)                                                   |                                                                                                                                   |
 | 1979 | [Find Greatest Common Divisor of Array](https://leetcode.com/problems/find-greatest-common-divisor-of-array)                                              |                                                        [![Java](assets/java.png)](src/FindGreatestCommonDivisorOfArray.java)                                                         |                                                                                                                                   |
 | 1984 | [Minimum Difference Between Highest and Lowest of K Scores](https://leetcode.com/problems/minimum-difference-between-highest-and-lowest-of-k-scores)      |                                                [![Java](assets/java.png)](src/MinimumDifferenceBetweenHighestAndLowestOfKScores.java)                                                |                                                                                                                                   |
-| 1991 | [Find the Middle Index in Array](https://leetcode.com/problems/find-the-middle-index-in-array)                                                            |                                                                                                                                                                                      |                                                                                                                                   |
+| 1991 | [Find the Middle Index in Array](https://leetcode.com/problems/find-the-middle-index-in-array)                                                            |                                                            [![Java](assets/java.png)](src/FindTheMiddleIndexInArray.java)                                                            |                                                                                                                                   |
 | 1995 | [Count Special Quadruplets](https://leetcode.com/problems/count-special-quadruplets)                                                                      |                                                                                                                                                                                      |                                                                                                                                   |
 | 2000 | [Reverse Prefix of Word](https://leetcode.com/problems/reverse-prefix-of-word)                                                                            |                                                                                                                                                                                      |                                                                                                                                   |
 | 2006 | [Count Number of Pairs With Absolute Difference K](https://leetcode.com/problems/count-number-of-pairs-with-absolute-difference-k)                        |                                                                                                                                                                                      |                                                                                                                                   |

Diff for: src/FindTheMiddleIndexInArray.java

@@ -0,0 +1,18 @@
+// https://leetcode.com/problems/find-the-middle-index-in-array
+// T: O(N)
+// S: O(1)
+
+import java.util.Arrays;
+
+public class FindTheMiddleIndexInArray {
+    public int findMiddleIndex(int[] nums) {
+        int leftSum = 0, rightSum = Arrays.stream(nums).sum() - nums[0];
+        if (leftSum == rightSum) return 0;
+        for (int index = 1 ; index < nums.length ; index++) {
+            rightSum -= nums[index];
+            leftSum += nums[index - 1];
+            if (leftSum == rightSum) return index;
+        }
+        return -1;
+    }
+}