LeetCode 2574. Left and Right Sum Differences

Given a 0-indexed integer array nums, find a 0-indexed integer array answer where:

• answer.length == nums.length.
• answer[i] = |leftSum[i] - rightSum[i]|.

Where:

• leftSum[i] is the sum of elements to the left of the index i in the array nums. If there is no such element, leftSum[i] = 0.
• rightSum[i] is the sum of elements to the right of the index i in the array nums. If there is no such element, rightSum[i] = 0.

Return the array answer.

Example 1:

Input: nums = [10,4,8,3]
Output: [15,1,11,22]
Explanation: The array leftSum is [0,10,14,22] and the array rightSum is [15,11,3,0].
The array answer is [|0 - 15|,|10 - 11|,|14 - 3|,|22 - 0|] = [15,1,11,22].

Example 2:

Input: nums = [1]
Output: [0]
Explanation: The array leftSum is [0] and the array rightSum is [0].
The array answer is [|0 - 0|] = [0].

Constraints:

• 1 <= nums.length <= 1000
• 1 <= nums[i] <= 105

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这题定义了leftSum为从左到右的prefix sum，rightSum为从右到左的prefix sum，求每个下标对应的Math.abs(leftSum - rightSum)。

最直观的做法就是用两个数组两个for loop分别求两边的prefix sum，然后再一个for loop做减法。

class Solution {
    public int[] leftRightDifference(int[] nums) {
        int len = nums.length;
        int[] leftSum = new int[len];
        int[] rightSum = new int[len];
        for (int i = 0; i < len; i++) {
            leftSum[i] = (i == 0) ? nums[i] : leftSum[i - 1] + nums[i];
        }
        for (int i = len - 1; i >= 0; i--) {
            rightSum[i] = (i == len - 1) ? nums[i] : rightSum[i + 1] + nums[i];
        }
        int[] result = new int[len];
        for (int i = 0; i < len; i++) {
            result[i] = Math.abs(leftSum[i] - rightSum[i]);
        }
        return result;
    }
}

然后看到有更简洁的方法，不需要额外的leftSum和rightSum数组就可以了。思路是先求出整个数组的sum。然后再从左往右for loop，sum - nums[i]其实就是rightSum[i]（每次需要更新sum），此时再动态求leftSum就可以了。

class Solution {
    public int[] leftRightDifference(int[] nums) {
        int len = nums.length;
        int sum = 0;
        for (int i = 0; i < len; i++) {
            sum += nums[i];
        }
        int[] result = new int[len];
        int leftSum = 0;
        for (int i = 0; i < len; i++) {
            int curr = nums[i];
            sum -= nums[i];
            result[i] = Math.abs(leftSum - sum);
            leftSum += nums[i];
        }
        return result;
    }
}