每日一題（day4）

Leetcode004

Median of Two Sorted Arrays問題
知識更新：求k小值、遞歸

private:
    double findKth(vector<int>& nums1, int i, vector<int>& nums2, int j, int k)
    {
        int len1 = nums1.size() - i;        
        int len2 = nums2.size() - j;
        if (len1 > len2) return findKth(nums2, j, nums1, i, k);
        if (!len1) return nums2[k - 1];
        if (k == 1) return min(nums1[i], nums2[j]);     
        int cur1, cur2;
        len1 < k / 2 ? cur1 = i + len1 - 1 : cur1 = i + k / 2 - 1;
        cur2 = j + k - (cur1 - i + 1) - 1;
        if (nums1[cur1] < nums2[cur2]) 
            return findKth(nums1, cur1 + 1, nums2, j, k - (cur1 - i + 1));
        if (nums1[cur1] > nums2[cur2])
            return findKth(nums1, i, nums2, cur2 + 1, k - (cur2 - j + 1));
        return nums1[cur1];
    }
public:
    double findMedianSortedArrays(vector<int>& nums1, vector<int>& nums2) { 
        int total_len = nums1.size() + nums2.size();
        if (total_len & 1) return findKth(nums1, 0, nums2, 0, total_len / 2 + 1);
        else return (findKth(nums1, 0, nums2, 0, total_len / 2 + 1) 
            + findKth(nums1, 0, nums2, 0, total_len / 2)) / 2;
    }

注意問題：遞歸時if判斷條件的先後順序，索引值與序號的轉換