88. Merge Sorted Array

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]

從數列的尾巴往前填回來。

void merge(vector<int>& nums1, int m, vector<int>& nums2, int n) { // time: O(m + n); space: O(1)
    int i = m - 1, j = n - 1;
    for (int k = m + n - 1; k >= 0; --k) {
        if (i >= 0 && j >= 0) {
            if (nums1[i] > nums2[j]) {
                nums1[k] = nums1[i--];
            } else {
                nums1[k] = nums2[j--];
            }
        } 
        // else if (i >= 0) {
        //      nums1[k] = nums1[i--];
        // }
        else if (j >= 0) {
            nums1[k] = nums2[j--];
        }
    }
}

void merge(vector<int>& nums1, int m, vector<int>& nums2, int n) { // time: O(m + n); space: O(1)
    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--];
    }
}