MaximumSubarray-LeetCode

problem

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.

Example:

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

Follow up:

If you have figured out the O(n) solution, try coding another solution using the divide and conquer approach, which is more subtle.

key

我們定義一個和爲第一位數，然後用curSum來保存遞增量

ps ans–>answer cur–>cursor

solution

class Solution {
    public int maxSubArray(int[] nums) {
        int ans=nums[0], curSum=0;
        
        for (int i=0; i<nums.length; i++) {
            curSum = curSum + nums[i];
            ans = Math.max(ans, curSum);
            curSum = Math.max(0, curSum);
        }
        
        return ans;
    }
}

perfect

class Solution {
    public int maxSubArray(int[] nums) {
        int dp = nums[0], maxSum=nums[0];
        for (int i=1; i<nums.length; i++) {
            dp = dp<0?nums[i]:nums[i]+dp;
            maxSum=Math.max(maxSum, dp);
        }
        return maxSum;
    }
}