p398

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LeetCode p398 Random Pick Index 题解

1.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);

题意：

实现一个数据结构，对于查找的数，随机返回对应的某个下标。

2.解题思路:

见代码

3.代码

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public class Solution {
    public HashMap<Integer, List<Integer>> hashMap;
    public Solution(int[] nums) {
        hashMap = new HashMap<Integer, List<Integer>>();
		for (int i = 0; i < nums.length; i++) {
			if (hashMap.containsKey(nums[i])) {
				List<Integer> list = hashMap.get(nums[i]);
				list.add(i);
			} else {
				List<Integer> list = new ArrayList<Integer>();
				list.add(i);
				hashMap.put(nums[i], list);
			}

		}
    }
    
    public int pick(int target) {
        if (!hashMap.containsKey(target))
			return -1;
		List<Integer> list = hashMap.get(target);
		int len = list.size();
		Random random = new Random();
		int i = random.nextInt(len);
		return list.get(i);
    }
}

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

4.一些总结：