[Leetcode] Merge Sorted Array

题目：

Given two sorted integer arrays A and B, merge B into A as one sorted array.

Note:

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

思路：从后向前merge即可，注意数组变空的判断。

class Solution {
public:
    void merge(int A[], int m, int B[], int n) {
        int a_ptr = m - 1;
        int b_ptr = n - 1;
        for (int i = m + n - 1; i >= 0; --i) {
            if (a_ptr < 0) {   // A[] has been empty
                A[i] = B[b_ptr];
                b_ptr--;
            } else if (b_ptr < 0) {   // B[] has been empty
                A[i] = A[a_ptr];
                a_ptr--;
            } else if (A[a_ptr] > B[b_ptr]) {
                A[i] = A[a_ptr];
                a_ptr--;
            } else {
                A[i] = B[b_ptr];
                b_ptr--;
            }
        }
    }
};

总结：复杂度O(m + n).