LeetCode-in-Java
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
Solution
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;
}
}