ID | Title | Difficulty | |
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398. Random Pick Index
Medium
LeetCode
Hash Table, Math, Reservoir Sampling, Randomized
Problem
Given an array of integers with possible duplicates, randomly output the index of a given target number. You can assume that the given target number must exist in the array.
Note: The array size can be very large. Solution that uses too much extra space will not pass the judge.
Example:
int[] nums = new int[] {1,2,3,3,3};
Solution solution = new Solution(nums);
// pick(3) should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.
solution.pick(3);
// pick(1) should return 0. Since in the array only nums[0] is equal to 1.
solution.pick(1);
Code
class Solution {
int[] nums;
Random rand;
public Solution(int[] nums) {
this.nums = nums;
this.rand = new Random();
}
public int pick(int target) {
int res = -1;
int count = 0;
for (int i = 0; i < nums.length; i++) {
if (nums[i] != target)
continue;
if (rand.nextInt(++count) == 0) {
res = i;
}
}
return res;
}
}
class Solution {
HashMap<Integer, List<Integer>> map;
Random rand;
public Solution(int[] nums) {
rand = new Random();
map = new HashMap<>();
for (int i = 0; i < nums.length; ++i) {
if (!map.containsKey(nums[i])) {
map.put(nums[i], new ArrayList<>());
}
map.get(nums[i]).add(i);
}
}
public int pick(int target) {
int size = map.get(target).size();
return map.get(target).get(rand.nextInt(size));
}
}
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399. Evaluate Division