LeetCode 398: Random Pick Index

转载于 2017-08-31 13:26:00 发布 · 63 阅读

CC 4.0 BY-SA版权

原文链接：http://www.cnblogs.com/shuashuashua/p/7457848.html

本文介绍了一种使用哈希映射和随机数生成器实现的随机索引解决方案。该方案能够根据目标值返回数组中该值的一个随机索引位置。通过维护两个哈希表分别记录每个值首次出现的位置和它们连续出现的长度，可以有效地实现这一功能。

class Solution {

    Map<Integer, Integer> startIndex;
    Map<Integer, Integer> lengths;
    Random random;
    public Solution(int[] nums) {
        random = new Random();
        startIndex = new HashMap<>();
        lengths = new HashMap<>();
        int length = 1;
        int start = nums[0];
        startIndex.put(nums[0], 0);
        for (int i = 1; i < nums.length; i++) {
            if (start != nums[i]) {
                lengths.put(start, length);
                length = 1;
                start = nums[i];
                startIndex.put(nums[i], i);
            } else{
                length++;
            }
        }
        lengths.put(start, length);
    }
    
    public int pick(int target) {
        if (!startIndex.containsKey(target)) {
            return 0;
        }
        
        return startIndex.get(target) + random.nextInt(lengths.get(target));
    }
}

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