Random Pick Index

update Aug 7,2017 18:01

LeetCode

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);

Basic Idea:

和上一题的思路相同。

Code:

Python Code:

    class Solution(object):

        def __init__(self, nums):
            """
            :type nums: List[int]
            :type numsSize: int
            """
            self.nums = nums


        def pick(self, target):
            """
            :type target: int
            :rtype: int
            """
            count = 0
            res = float('INF')
            for i in range(len(self.nums)):
                if self.nums[i] == target:
                    if (random.randint(0, count) == 0):
                        res = i
                    count += 1
            return res

Java Code