[LeetCode] 724. Find Pivot Index

轉自LeetCode

Given an array of integers nums, write a method that returns the "pivot" index of this array.

We define the pivot index as the index where the sum of the numbers to the left of the index is equal to the sum of the numbers to the right of the index.

If no such index exists, we should return -1. If there are multiple pivot indexes, you should return the left-most pivot index.

Example 1:

Input: 
nums = [1, 7, 3, 6, 5, 6]
Output: 3
Explanation: 
The sum of the numbers to the left of index 3 (nums[3] = 6) is equal to the sum of numbers to the right of index 3.
Also, 3 is the first index where this occurs.

Example 2:

Input: 
nums = [1, 2, 3]
Output: -1
Explanation: 
There is no index that satisfies the conditions in the problem statement.

Note:

The length of nums will be in the range [0, 10000].

Each element nums[i] will be an integer in the range [-1000, 1000].

<Solution>

想法如下

• 先計算出整個 array 的總和 totalSum

• 計算目前在 index 左邊的總和 leftSum，而在 index 右邊的總和就會等於 totalSum - leftSum - nums[index]。這時候再檢查是否左右邊的總和相等，是的話就回傳 index

code 如下

Java