maintain invariance: "max_so_far" and "max_current".
Calculate the max till current number. it only depends if the previous max is "+" or "-";

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