class Solution 
{
public:
    //双指针算法
    int findRepeatNum(vector<int>& nums) 
    {
        sort(nums.begin(),nums.end());
        int n=nums.size();
        int left=0,right=n-1;
        while(nums[left]!=nums[left+1])left++;
        while(nums[right]!=nums[right-1])right--;
        return nums[right];//返回right和left都一样
    }
};