Leetcode:random Pick Index

Given An array of integers with possible duplicates, randomly output the index of a Given target number. You can assume this given target number must exist in the array. Note:the array size can be very large. Solution that uses too much extra space would not pass the judge. Example:int[] Nums =New int[] {1,2,3,3,3}; Solution Solution=Newsolution (nums);//pick (3) should return either index 2, 3, or 4 randomly. Each index should has 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);

Reservior Sampling

1  Public classSolution {2     int[] arr;3 random random;4     5 6      PublicSolution (int[] nums) {7arr =nums;8Random =NewRandom ();9     }Ten      One      Public intPickinttarget) { A         intCount = 0; -         intindex = 0; -          for(inti=0; i<arr.length; i++) { the             if(Arr[i] = =target) { -count++; -                 if(Random.nextint (count) = = 0) { -index =i; +                 } -             } +         } A         returnindex; at     } - } -  - /** - * Your Solution Object would be instantiated and called as such: - * Solution obj = new solution (nums); in * int param_1 = Obj.pick (target); -  */

Leetcode:random Pick Index