java:
class Solution {
       public int removeDuplicates(int[] nums) {
        if (nums.length == 0) return 0;
        int i = 0;
        for (int j = 1; j < nums.length; j++) {
            if (nums[j] != nums[i]) {
                i++;
                nums[i] = nums[j];      //相当与在把不重复往前面移动
            }
        }
        return i + 1;
    }
}

class Solution {
public:
    int removeDuplicates(vector<int>& nums) {
         nums.erase(unique(nums.begin(),nums.end()),nums.end());
        return nums.size();
    }
};