53. Maximum Subarray

题目描述：(剑指offer原题31)

https://leetcode.com/problems/maximum-subarray/

解决方法：

https://leetcode.com/problems/maximum-subarray/discuss/20200/Share-my-solutions-both-greedy-and-divide-and-conquer

optim_code(c++):

运行时间12ms:

class Solution {
public:
    int maxSubArray(vector& nums) {
        if(nums.empty())
            return 0;
        int nCurSum  = 0 ;
        int nGreatestSum = INT32_MIN;
        for( int i = 0;i< nums.size();i++){
            if(nCurSum<=0){
                nCurSum = nums.at(i);
            }else{
                nCurSum += nums.at(i);
            }
            if(nCurSum > nGreatestSum)
                nGreatestSum = nCurSum;
        }
        return nGreatestSum;
    }
};

运行时间8ms：

class Solution {
public:
    int maxSubArray(vector& nums) {
        if(nums.empty())
            return 0;
        int nCurSum  = 0 ;
        int nGreatestSum = INT32_MIN;
        for( int i = 0;i< nums.size();i++){
            nCurSum = max(nums.at(i), nCurSum +nums.at(i));
            nGreatestSum = max(nCurSum, nGreatestSum);
        }
        return nGreatestSum;
    }
};

运行时间4ms：

class Solution {
public:
    int maxSubArray(vector& nums) {
        if(nums.empty())
            return 0;
        int res = nums[0], sum = res;
        for(int i = 1; i < nums.size(); i++) {
            sum = sum > 0 ? sum+nums[i]: nums[i];
            res = max(sum,res);
        }
        return res;
    }
};

心得：

以上解法，思路都是一样的，但是算法书写形式影响效率
内置函数max，比手动判断效率要高，当然if判断并不是得到最大值的最优方法
三目运算表达式比max函数效率高