从前往后进行遍历

需要开辟空间

import java.util.*;
public class Solution {
    public void merge(int A[], int m, int B[], int n) {
        int tmp[] = new int[n + m];
        int i = 0,j = 0;
        int idx = 0;
        while(i < m && j < n){
            if(A[i] <= B[j]){
                tmp[idx++] = A[i++];
            }else {
                tmp[idx++] = B[j++];
            }
        }
        while(i < m){tmp[idx++] = A[i++];}
        while(j < n){tmp[idx++] = B[j++];}
        for(i = 0;i < n + m;i++){
            A[i] = tmp[i];
        }
    }
}

从后往前进行遍历

不需要开辟空间

import java.util.*;
public class Solution {
    public void merge(int A[], int m, int B[], int n) {
        int idx = n + m - 1;
        int i = m - 1;
        int j = n - 1;
        while(i >= 0 && j >= 0){
            if(A[i] >= B[j])A[idx--] = A[i--];
            else {
                A[idx--] = B[j--];
            }
        }
        while(i >= 0)A[idx--] = A[i--];
        while(j >= 0)A[idx--] = B[j--];
    }
}