LeetCode88. Merge Sorted Array（模拟）

LeetCode88. Merge Sorted Array（模拟）

• Easy
• Accepted：318,463
• Submissions：921,821

Given two sorted integer arrays nums1 and nums2, merge nums2 into nums1 as one sorted array.

Note:

• The number of elements initialized in nums1 and nums2 are m and n respectively.
• You may assume that nums1 has enough space (size that is greater or equal to m + n) to hold additional elements from nums2.

Example:

Input:
nums1 = [1,2,3,0,0,0], m = 3
nums2 = [2,5,6],       n = 3

Output: [1,2,2,3,5,6]

链接

https://leetcode.com/problems/merge-sorted-array/

题意

给定两个有序的数组nums1和nums2，将nums2合并到nums1中并保持有序，保证nums1有足够的空间来存储两个数组的元素。

题解

• 两个有序数组的合并其实就是归并排序中的合并操作。从头到尾遍历的话，就需要再开一个辅助数组，因为直接操作的话num1数组中的元素可能会被覆盖。时间复杂度$O(n+m)$，空间复杂度$O(n+m)$
• 可以选择从后往前遍历，这样就无须额外的辅助数组，时间复杂度$O(n+m)$，空间复杂度$O(1)$，刚好可以用上题目中的特殊条件

代码

• Runtime: 4 ms, faster than 86.91% of C++ online submissions for Merge Sorted Array.
• Memory Usage: 790.5 KB, less than 16.20% of C++ online submissions for Merge Sorted Array.

class Solution {
public:
    void merge(vector<int>& nums1, int m, vector<int>& nums2, int n) {
        int i = 0, j = 0, tmp;
        vector <int> numsAns; // 辅助数组
        numsAns.clear();
        while (i < m && j < n)
        {
            tmp = nums1[i] <= nums2[j] ? nums1[i++] : nums2[j++];
            numsAns.push_back(tmp);
        }
        while (i < m)
            numsAns.push_back(nums1[i++]);
        while (j < n)
            numsAns.push_back(nums2[j++]);
        for (int k = 0; k < m + n; ++k) // 还原
            nums1[k] = numsAns[k];
    }
};

• Runtime: 4 ms, faster than 86.91% of C++ online submissions for Merge Sorted Array.
• Memory Usage: 774.1 KB, less than 85.65% of C++ online submissions for Merge Sorted Array.

class Solution {
public:
    void merge(vector<int>& nums1, int m, vector<int>& nums2, int n) {
        // 从后往前插入
        int i = m - 1, j = n - 1, k = m + n - 1;
        while (i >= 0 && j >= 0)
            nums1[k--] = (nums1[i] > nums2[j] ? nums1[i--] : nums2[j--]);
        while (j >= 0)
            nums1[k--] = nums2[j--];
    }
};

The end.
2019年2月2日 星期六