[Leetcode]398. Random Pick Index

题目：

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);

代码：

1.第一种写法

public class Solution {

    private int[]nums;
    Random random =new Random();
    public Solution(int[] nums) {
       this.nums = nums;
    }
    
    public int pick(int target) {
        ArrayList<Integer>list = new ArrayList<Integer>();
        for(int i = 0 ; i < nums.length;i++){
            if(nums[i]==target){
                list.add(i);
            }
        }
        //把目标值存放在list中，然后等概率的返回
        return list.get(random.nextInt(list.size()));
    }
}


/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(nums);
 * int param_1 = obj.pick(target);

 */

2.第二种写法：

1. public class Solution { // 417ms  
2.     private int[] nums;  
3.     private Random random;  
4.   
5.     public Solution(int[] nums) {  
6.         this.nums = nums;  
7.         this.random = new Random();  
8.     }  
9.   
10.     public int pick(int target) {  
11.         int result = -1;  
12.         int count = 1;  
13.         for (int i = 0; i < nums.length; i++) {  
14.             if (nums[i] == target) {  
15.                 if (random.nextInt(count) == 0) {  
16.                     result = i;  
17.                 }   
18.                 count++;  
19.             }  
20.         }  
21.         return result;  
22.     }  
23. }  
24.   
25. /** 
26.  * Your Solution object will be instantiated and called as such: 
27.  * Solution obj = new Solution(nums); 
28.  * int param_1 = obj.pick(target); 
29.  */