zoukankan      html  css  js  c++  java
  • [LeetCode] Longest Consecutive Sequence

    This problem is not very intuitive at first glance. However, the final idea should be very self-explanatory. You visit each element in nums, and then find its left and right neighbors and extend the length accordingly. The time complexity is O(n) if we guard from visiting the same element again.

    You can implement it using an unordered_set or unordered_map.

    The code is as follows. The last one is taken from this page which includes a nice explanation in the follong answers.

     1 // O(n) solution using unordered_set
     2 int longestConsecutive(vector<int>& nums) {
     3     unordered_set<int> copy(nums.begin(), nums.end());
     4     unordered_set<int> filter;
     5     int len = 0;
     6     for (int i = 0; i < nums.size(); i++) {
     7         if (filter.find(nums[i]) != filter.end()) continue;
     8         int l = nums[i] - 1, r = nums[i] + 1;
     9         while (copy.find(l) != copy.end()) filter.insert(l--);
    10         while (copy.find(r) != copy.end()) filter.insert(r++);
    11         len = max(len, r - l - 1);
    12     }
    13     return len;
    14 }
    15 
    16 // O(n) solution using unordered_map
    17 int longestConsecutive(vector<int>& nums) {
    18     unordered_map<int, int> mp;
    19     int len = 0;
    20     for (int i = 0; i < nums.size(); i++) {
    21         if (mp[nums[i]]) continue;
    22         mp[nums[i]] = 1;
    23         if (mp.find(nums[i] - 1) != mp.end())
    24             mp[nums[i]] += mp[nums[i] - 1];
    25         if (mp.find(nums[i] + 1) != mp.end())
    26             mp[nums[i]] += mp[nums[i] + 1];
    27         mp[nums[i] - mp[nums[i] - 1]] = mp[nums[i]]; // left boundary
    28         mp[nums[i] + mp[nums[i] + 1]] = mp[nums[i]]; // right boundary
    29         len = max(len, mp[nums[i]]);
    30     }
    31     return len;
    32 }
    33 
    34 // O(n) super-concise solution (merging the above cases)
    35 int longestConsecutive(vector<int>& nums) {
    36     unordered_map<int, int> mp;
    37     int len = 0;
    38     for (int i = 0; i < nums.size(); i++) {
    39         if (mp[nums[i]]) continue;
    40         len = max(len, mp[nums[i]] = mp[nums[i] - mp[nums[i] - 1]] = mp[nums[i] + mp[nums[i] + 1]] = mp[nums[i] - 1] + mp[nums[i] + 1] + 1);
    41     }
    42     return len;
    43 }
  • 相关阅读:
    【Java学习】向上和向下转型
    【Java学习】java抽象类的作用
    【Java学习】追踪
    【Java学习】@Override 解释
    【Java学习】Java 中带包(创建及引用)的类的编译与调试
    【Java学习】相关基础算法
    【Java学习】import和import static的区别
    【testNG学习】testng.xml文件
    Reshape the Matrix
    Distribute Candies
  • 原文地址:https://www.cnblogs.com/jcliBlogger/p/4572028.html
Copyright © 2011-2022 走看看