【LeetCode】53. Maximum Subarray 解題報告

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53. Maximum Subarray / Easy

Given an integer array nums, find the contiguous subarray (containing at least one number) which has the largest sum and return its sum.

A subarray is a contiguous part of an array.

Example 1:

Input: nums = [-2,1,-3,4,-1,2,1,-5,4]
Output: 6
Explanation: [4,-1,2,1] has the largest sum = 6.

Example 2:

Input: nums = [1]
Output: 1

Example 3:

Input: nums = [5,4,-1,7,8]
Output: 23

Constraints:

  • 1 <= nums.length <= 3 * 10^4
  • -10^5 <= nums[i] <= 10^5

Solution: Map

思路

題目不需要找出子陣列，只要找出最大的和，所以直接一個一個加起來最快。
如果目前的總和小於目前的數字，代表前面加起來是負的，直接把目前的數字設成總和就可以了。

效能

Complexity

  • Time Complexity: O(N)
  • Space Complexity: O(1)

LeetCode Result

Code

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class Solution {
public:
    int maxSubArray(vector<int>& nums) {
        int sum = 0;
        int maxNum = INT_MIN;
        for(int num: nums) {
            sum += num;
            if(num > sum) sum = num;
            maxNum = max(maxNum, sum);
        }
        return maxNum;
    }
};