依次比较移动

class Solution {
public:
    int removeElement(vector<int>& nums, int val) {
        if(nums.size() == 0) return 0;
        int j = 0;
        for(int i = 0; i < nums.size(); i++){
           if (nums[i] != val) nums[j++] = nums[i];
        }
        return j;
    }
};