Added task 1991.

Author: javadev

src/main/java/g1901_2000/s1991_find_the_middle_index_in_array/Solution.java

@@ -0,0 +1,31 @@
+package g1901_2000.s1991_find_the_middle_index_in_array;
+
+// #Easy #Array #Prefix_Sum #2022_05_18_Time_0_ms_(100.00%)_Space_40.2_MB_(97.29%)
+
+public class Solution {
+ // TC : O(1), SC: (1)
+ public int findMiddleIndex(int[] nums) {
+ // find the sum of all numbers in the array
+ int sum = 0;
+ for (int n : nums) {
+ sum += n;
+ }
+ // consider leftSum = 0, rightSum = sum
+ int leftSum = 0;
+ int rightSum = sum;
+ /*
+ Traverse the array: At each index, subtract the element from rightSum and
+ check if leftSum equals rightSum. If they do, return the index.
+ Otherwise, add the number at current index to the leftSum and traverse further.
+ */
+ for (int i = 0; i < nums.length; i++) {
+ rightSum -= nums[i];
+ if (leftSum == rightSum) {
+ return i;
+ }
+ leftSum += nums[i];
+ }
+ // index not found, return -1
+ return -1;
+ }
+}

src/main/java/g1901_2000/s1991_find_the_middle_index_in_array/readme.md

@@ -0,0 +1,46 @@
+1991\. Find the Middle Index in Array
+
+Easy
+
+Given a **0-indexed** integer array `nums`, find the **leftmost** `middleIndex` (i.e., the smallest amongst all the possible ones).
+
+A `middleIndex` is an index where `nums[0] + nums[1] + ... + nums[middleIndex-1] == nums[middleIndex+1] + nums[middleIndex+2] + ... + nums[nums.length-1]`.
+
+If `middleIndex == 0`, the left side sum is considered to be `0`. Similarly, if `middleIndex == nums.length - 1`, the right side sum is considered to be `0`.
+
+Return _the **leftmost**_ `middleIndex` _that satisfies the condition, or_ `-1` _if there is no such index_.
+
+**Example 1:**
+
+**Input:** nums = [2,3,-1,8,4]
+
+**Output:** 3
+
+**Explanation:** The sum of the numbers before index 3 is: 2 + 3 + -1 = 4
+
+The sum of the numbers after index 3 is: 4 = 4 
+
+**Example 2:**
+
+**Input:** nums = [1,-1,4]
+
+**Output:** 2
+
+**Explanation:** The sum of the numbers before index 2 is: 1 + -1 = 0
+
+The sum of the numbers after index 2 is: 0 
+
+**Example 3:**
+
+**Input:** nums = [2,5]
+
+**Output:** -1
+
+**Explanation:** There is no valid middleIndex. 
+
+**Constraints:**
+
+* `1 <= nums.length <= 100`
+* `-1000 <= nums[i] <= 1000`
+
+**Note:** This question is the same as 724

src/test/java/g1901_2000/s1991_find_the_middle_index_in_array/SolutionTest.java

@@ -0,0 +1,23 @@
+package g1901_2000.s1991_find_the_middle_index_in_array;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class SolutionTest {
+ @Test
+ void findMiddleIndex() {
+ assertThat(new Solution().findMiddleIndex(new int[] {2, 3, -1, 8, 4}), equalTo(3));
+ }
+
+ @Test
+ void findMiddleIndex2() {
+ assertThat(new Solution().findMiddleIndex(new int[] {1, -1, 4}), equalTo(2));
+ }
+
+ @Test
+ void findMiddleIndex3() {
+ assertThat(new Solution().findMiddleIndex(new int[] {2, 5}), equalTo(-1));
+ }
+}