Add some code

leetcode/problems/java/find-k-th-smallest-pair-distance.java

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+
+import java.util.Arrays;
+
+class Solution {
+  public int smallestDistancePair(int[] numbers, int k) {
+    Arrays.sort(numbers);
+    int minDist = 0;
+    int maxDistance = numbers[numbers.length - 1] - numbers[0];
+
+    while (minDist < maxDistance) {
+      int mid = minDist + (maxDistance - minDist) / 2;
+      int count = countPairsWithinDistance(numbers, mid);
+      if (count < k)
+        minDist = mid + 1;
+      else
+        maxDistance = mid;
+    }
+    return minDist;
+  }
+
+  private int countPairsWithinDistance(int[] numbers, int targetDistance) {
+    int count = 0;
+    int left = 0;
+    for (int right = 1; right < numbers.length; right++) {
+      while (numbers[right] - numbers[left] > targetDistance)
+        left++;
+      count += right - left;
+    }
+    return count;
+  }
+}