Given an array and a value, remove all instances of that value in place and return the new length.

The order of elements can be changed. It doesn't matter what you leave beyond the new length.

思路：双指针，指针 i 遍历，另一指针 j 指向下一个非elem的位置，返回 j.

public class Solution {    public int removeElement(int[] A, int elem) {        int j=0;        for(int i=0;i