#64 Merge Sorted Array

題目描述：

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

 Notice

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 n respectively.

Have you met this question in a real interview? 

Yes

Example

A = [1, 2, 3, empty, empty], B = [4, 5]

After merge, A will be filled as [1, 2, 3, 4, 5]

題目思路：

這題用two pointers做。如果要做成in-place的，爲了避免有用信息被overwrite，需要在A中從後往前開始寫。

Mycode（AC = 32ms）：

class Solution {
public:
    /**
     * @param A: sorted integer array A which has m elements, 
     *           but size of A is m+n
     * @param B: sorted integer array B which has n elements
     * @return: void
     */
    void mergeSortedArray(int A[], int m, int B[], int n) {
        // write your code here
        if (n == 0) {
            return;
        }
        
        int idxA = m - 1, idxB = n - 1;
        for (int i = m + n - 1; i >= 0; i--) {
            if (idxB < 0 || A[idxA] > B[idxB]) {
                A[i] = A[idxA];
                idxA--;
            }
            else {//(idxA < 0 || A[idxA] <= B[idxB]) {
                A[i] = B[idxB];
                idxB--;
            }
        }
    }
};