719. Find K-th Smallest Pair Distance

 JAVA

• class Solution {
      public int smallestDistancePair(int[] nums, int k) {
          Arrays.sort(nums);
          int left = 0, right = nums[nums.length - 1] - nums[0];
          while (left < right) {
              int mid = (left + right) >> 1;
              if (count(mid, nums) >= k) {
                  right = mid;
              } else {
                  left = mid + 1;
              }
          }
          return left;
      }
  
      private int count(int dist, int[] nums) {
          int cnt = 0;
          for (int i = 0; i < nums.length; ++i) {
              int left = 0, right = i;
              while (left < right) {
                  int mid = (left + right) >> 1;
                  int target = nums[i] - dist;
                  if (nums[mid] >= target) {
                      right = mid;
                  } else {
                      left = mid + 1;
                  }
              }
              cnt += i - left;
          }
          return cnt;
      }
  }

C++

• class Solution {
  public:
      int smallestDistancePair(vector<int>& nums, int k) {
          sort(nums.begin(), nums.end());
          int left = 0, right = nums.back() - nums.front();
          while (left < right) {
              int mid = (left + right) >> 1;
              if (count(mid, k, nums) >= k)
                  right = mid;
              else
                  left = mid + 1;
          }
          return left;
      }
  
      int count(int dist, int k, vector<int>& nums) {
          int cnt = 0;
          for (int i = 0; i < nums.size(); ++i) {
              int target = nums[i] - dist;
              int j = lower_bound(nums.begin(), nums.end(), target) - nums.begin();
              cnt += i - j;
          }
          return cnt;
      }
  };