leetcode 217. Contains Duplicate 287. Find the Duplicate Number

class Solution {
public:
    bool containsDuplicate(vector<int>& nums) {
        int length = nums.size();
        if(length <= 0)
            return false;
        sort(nums.begin(),nums.end());
        for(int i = 1;i < length;i++){
            if(nums[i] == nums[i-1])
                return true;
        }
        return false;
    }
};

class Solution {
public:
    int findDuplicate(vector<int>& nums) {
        int length = nums.size();
        if(length <= 0)
            return -1;
        int start = 1;
        int end = length - 1;
        while(start < end){
            int mid = (start + end)/2;
            int count = 0;
            for(int i = 0;i < length;i++){
                if(nums[i] <= mid)
                    count++;
            }
            if(count > mid)
                end = mid;
            else
                start = mid + 1;
        }
        return start;
    }
};