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[LeetCode] Intersection of Two Arrays II 两个数组相交之二

 

Given two arrays, write a function to compute their intersection.

Example 1:

Input: nums1 = [1,2,2,1], nums2 = [2,2]
Output: [2,2]

Example 2:

Input: nums1 = [4,9,5], nums2 = [9,4,9,8,4]
Output: [4,9]

Note:

  • Each element in the result should appear as many times as it shows in both arrays.
  • The result can be in any order.

Follow up:

  • What if the given array is already sorted? How would you optimize your algorithm?
  • What if nums1's size is small compared to nums2's size? Which algorithm is better?
  • What if elements of nums2 are stored on disk, and the memory is limited such that you cannot load all elements into the memory at once?
 
 
这道题是之前那道Intersection of Two Arrays的拓展,不同之处在于这道题允许我们返回重复的数字,而且是尽可能多的返回,之前那道题是说有重复的数字只返回一个就行。那么这道题我们用哈希表来建立nums1中字符和其出现个数之间的映射, 然后遍历nums2数组,如果当前字符在哈希表中的个数大于0,则将此字符加入结果res中,然后哈希表的对应值自减1,参见代码如下:

 

解法一:

class Solution {
public:
    vector<int> intersect(vector<int>& nums1, vector<int>& nums2) {
        unordered_map<int, int> m;
        vector<int> res;
        for (auto a : nums1) ++m[a];
        for (auto a : nums2) {
            if (m[a]-- > 0) res.push_back(a);
        }
        return res;
    }
};

 

再来看一种方法,这种方法先给两个数组排序,然后用两个指针分别指向两个数组的起始位置,如果两个指针指的数字相等,则存入结果中,两个指针均自增1,如果第一个指针指的数字大,则第二个指针自增1,反之亦然,参见代码如下:

 

解法二:

class Solution {
public:
    vector<int> intersect(vector<int>& nums1, vector<int>& nums2) {
        vector<int> res;
        int i = 0, j = 0;
        sort(nums1.begin(), nums1.end());
        sort(nums2.begin(), nums2.end());
        while (i < nums1.size() && j < nums2.size()) {
            if (nums1[i] == nums2[j]) {
                res.push_back(nums1[i]);
                ++i; ++j;
            } else if (nums1[i] > nums2[j]) {
                ++j;
            } else {
                ++i;
            }
        }
        return res;
    }
};

 

类似题目:

Intersection of Two Arrays

 

参考资料:

https://leetcode.com/problems/intersection-of-two-arrays-ii/

https://leetcode.com/problems/intersection-of-two-arrays-ii/discuss/82269/Short-Python-C%2B%2B

https://leetcode.com/problems/intersection-of-two-arrays-ii/discuss/82241/AC-solution-using-Java-HashMap

https://leetcode.com/problems/intersection-of-two-arrays-ii/discuss/82263/C%2B%2B-hash-table-solution-and-sort-%2B-two-pointers-solution-with-time-and-space-complexity

 

LeetCode All in One 题目讲解汇总(持续更新中...) 

posted @ 2016-05-27 06:21 Grandyang 阅读(...) 评论(...) 编辑 收藏