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_398.java
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_398.java
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import java.util.Random;
/**
* LeetCode 398 - Random Pick Index
* <p>
* 1. Sort indices [0, n) by the corresponding value.
* 2. For each pick query, we do two binary searches to find the left and right boundaries and then throw a die to
* return a random index.
* <p>
* O(n log n) build time
* O(log n) query time
*/
public class _398 {
Random rand;
int[] rank;
int[] nums;
private void quickSort(int l, int r) {
int i = l, j = r, x = rank[rand.nextInt(r - l + 1) + l];
do {
while (nums[rank[i]] < nums[x]) i++;
while (nums[rank[j]] > nums[x]) j--;
if (i <= j) {
int tmp = rank[i];
rank[i] = rank[j];
rank[j] = tmp;
i++;
j--;
}
}
while (i <= j);
if (i < r) quickSort(i, r);
if (j > l) quickSort(l, j);
}
public _398(int[] nums) {
rand = new Random();
this.nums = nums;
rank = new int[nums.length];
for (int i = 0; i < rank.length; i++) rank[i] = i;
quickSort(0, rank.length - 1);
}
public int pick(int target) {
int left = -1, right = rank.length - 1;
while (left + 1 < right) {
int mid = (left + right) / 2;
if (nums[rank[mid]] < target) left = mid;
else right = mid;
}
int begin = right;
left = 0;
right = rank.length;
while (left + 1 < right) {
int mid = (left + right) / 2;
if (nums[rank[mid]] > target) right = mid;
else left = mid;
}
int end = left;
return rank[rand.nextInt(end - begin + 1) + begin];
}
}