E
1522013551
tags: Array, Hash Table
Unsorted array, 找出是否有duplicate elemenets: 必要条件是, 这两个element的index i,j 的大小最多相差k.
#### HashSet
- 很巧妙地根据k range地条件, 把HashSet里面的值控制在[i - k, i]
- 每次不断地往set里面加新元素, 从set里减去末尾index的元素
- 而set.add(x)如果遇到重复, 会return false.
- 一旦在这个length k 的 range里面, 有重复, 就符合条件.
- Time O(n)
#### HashTable
- 记录每个element value的index in the list
- 一旦有重复element重复, 就把整个list of indexes 端出来, 查看有没有符合条件的: (index - i) <= k
- Time O(nm), m = # of duplicates
#### 这两种做法的区别很有艺术感觉
- 方法1是限定选拔的candidate, 不合格就去掉, 那么一旦有符合条件的(duplicates), 那么一定中, 剩下的就不看了.
- 方法2是把符合条件的index找出来, 集中处理, 但是所有candidate都会选出来
- 就好像招人一样: 一种是遇到好的就停止; 第二种是看过所有人, 从中选拔最好的. 显然第一种更快.
```
/*
Given an array of integers and an integer k,
find out whether there are two distinct indices i and j in the array such that:
nums[i] = nums[j] and the absolute difference between i and j is at most k.
*/
/*
Thoughts:
1. Think of k as the upper limit of the range that we want to pick our i and j.
2. When i is in [0 ~ k], we can pick and j that's also in [0 ~ k]. This can be checked in a for loop, with a HashSet.
3. Once i pass k, we need to remove any value that's in range [0, i-k) from the set, because they are out of range.
They are no longer fit the condition to duplicate with nums[i], regardless if they are duplicates or not
Note: set.add(..) return false if the value already exist in set.
*/
class Solution {
public boolean containsNearbyDuplicate(int[] nums, int k) {
if (nums == null || nums.length == 0 || k <= 0) {
return false;
}
final Set set = new HashSet<>();
for (int i = 0; i < nums.length; i++) {
if (i > k) {
set.remove(nums[i - k - 1]);
}
if (!set.add(nums[i])) {
return true;
}
}
return false;
}
}
/*
Thoughts:
Store in hashmap. When there is a duplicate, check against k.
Though, quite slow: O(n * h), where h is the possible duplicates. In the extreme case when n = h, it becomes O(n^2)
*/
class Solution {
public boolean containsNearbyDuplicate(int[] nums, int k) {
if (nums == null || nums.length == 0 || k <= 0) {
return false;
}
final Map> map = new HashMap<>();
for (int i = 0; i < nums.length; i++) {
if (map.containsKey(nums[i])) {
for (int index : map.get(nums[i])) {
if (Math.abs(index - i) <= k) {
return true;
}
}
} else {
map.put(nums[i], new ArrayList<>());
}
map.get(nums[i]).add(i);
}
return false;
}
}
```