Lintcode: Minimum Subarray

C++

Divide-Conquer[71% passed]

class Solution {
public:
    /**
     * @param nums: a list of integers
     * @return: A integer denote the sum of minimum subarray
     */
    int minSubArray(vector<int> nums) {
        // write your code here
        int n = nums.size();
        if (n == 1) {
            return nums[0];
        }
        int *arr = nums.data();
        return helper(arr, n);
    }
    int helper(int *arr, int n) {
        // terminate
        if ( n == 1 ) {
            return arr[0];
        }
        // divide
        int mid = n>>1;
        int ans = min(helper(arr, mid), helper(arr + mid, n - mid));
        // conquer
        int now = arr[mid - 1], may = now;
        for (int i = mid-2; i >= 0; --i) {
            may = min(may, now += arr[i]);
        }
        now = may;
        for (int i = mid; i <= n; ++i) {
            may = min(may, now += arr[i]);
        }
        // return
        return min(may, ans);
    }
};

C++, dp

class Solution {
public:
    /**
     * @param nums: a list of integers
     * @return: A integer denote the sum of minimum subarray
     */
    int minSubArray(vector<int> nums) {
        // write your code here
        int n = nums.size();
        if (n == 1) {
            return nums[0];
        }
        vector<int> dp(n);
        dp[0] = nums[0];
        int ans = nums[0];
        for (int i = 1; i < n ; ++i) {
            dp[i] = min(nums[i], dp[i - 1]+nums[i]);
            ans = min(dp[i], ans);
        }
        return ans;
    }
};

C++,dp,O(1) space

class Solution {
public:
    /**
     * @param nums: a list of integers
     * @return: A integer denote the sum of minimum subarray
     */
    int minSubArray(vector<int> nums) {
        // write your code here
        int n = nums.size();
        if (n == 1) {
            return nums[0];
        }
        int endHere = nums[0];
        int ans = nums[0];
        for (int i = 1; i < n ; ++i) {
            endHere = min(nums[i], endHere+nums[i]);
            ans = min(endHere, ans);
        }
        return ans;
    }
};