找到K个最接近的元素-滑窗、二分

二分法：左闭右开

class Solution {
public:
    vector<int> findClosestElements(vector<int>& arr, int k, int x) {
        int left = 0;
        int right = arr.size() - k;
        while(left < right){
            int mid = left + (right - left) / 2;
            if(x - arr[mid] <= arr[mid + k] - x){
                right = mid;
            }
            else{
                left = mid + 1;
            }
        }
        return vector<int>(arr.begin() + left, arr.begin() + left + k);
    }
};

二分法：闭区间

class Solution {
public:
    vector<int> findClosestElements(vector<int>& arr, int k, int x) {
        int left = 0;
        int right = arr.size() - k - 1;
        while(left <= right){
            int mid = left + (right - left) / 2;
            if(x - arr[mid] > arr[mid + k] - x){
                left = mid + 1;
            }
            else{
                right = mid - 1;
            }
        }
        return vector<int>(arr.begin() + left, arr.begin() + left + k);
    }
};

从两边删除元素

class Solution {
public:
    vector<int> findClosestElements(vector<int>& arr, int k, int x) {
        list<int> num(arr.begin(), arr.end());
        while(num.size() > k){
            if(abs(num.front() - x) > abs(num.back() - x)){
                num.pop_front();
            }
            else{
                num.pop_back();
            }
        }
        return vector<int>(num.begin(), num.end());
    }
};

// Delete the the farther one of leftmost side and rightmost side element until k left
class Solution2 {
public:
    vector<int> findClosestElements(vector<int>& arr, int k, int x) {
        auto res = arr;
        while (res.size() > k) {
            if (x - res.front() > res.back() - x) {
                res.erase(res.begin());
            } else {
               res.pop_back();
            }
        }

        return res;
    }
};

滑窗：

// Sliding window
class Solution1 {
public:
    vector<int> findClosestElements(vector<int>& arr, int k, int x) {
        int minDelta = INT_MAX;

        int delta = 0;
        int startIdx = 0;
        for (int i = 0; i < k; i++) {
            delta += abs(arr[i] - x);
        }

        minDelta = min(delta, minDelta);
        for (int i = k; i < arr.size(); i++) {
           delta -= abs(arr[i - k] - x); 
           delta += abs(arr[i] - x);
           if (delta < minDelta) {
               minDelta = delta;
               startIdx = i - k + 1;
           }
        }

        return vector<int>(arr.begin() + startIdx, arr.begin() + startIdx + k);
    }
};