【Leetcode Summary】1-5题总结

转载于 2019-02-20 19:21:24 发布 · 198 阅读

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本文深入解析LeetCode上的经典算法题目，包括两数之和、相加链表、最长无重复子串、两数组中位数及最长回文子串等，提供高效算法思路与代码实现。

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1.TwoSum

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

solution

想到用map<int,int>来实现 first为数值，second为索引

对数组进行循环，每次查找map中是否有target - nums[i]数字存在。如果有返回索引

否则把当前数字加入map，继续循环。

时间复杂度为O{n}，（这里留一个坑如果map的find算法的时间复杂是O（1）的话）

感觉还是需要把STL源码剖析看一下，不然对map这些底层的实现效率都不是很了解

2.Add Twonum

u are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

首先这道题有可能一个数字为0 或者出现进位的情况

大题思路是：对两个链表进行循环，直到其中一个到尾。每次记录进位标志，以及清零标志。

下次计算高位时选择+1/不加。

注意 corner case：5+5 循环只循环一次但是有进位出现，所以循环结束后还需处理进位符号

注意 next指针可能为空要确认有效才能进行下一步

3. Longest Substring Without Repeating Characters

Given a string, find the length of the longest substring without repeating characters.

Example 1:

Input: "abcabcbb"
Output: 3 
Explanation: The answer is "abc", with the length of 3.

Example 2:

Input: "bbbbb"
Output: 1
Explanation: The answer is "b", with the length of 1.

Example 3:

Input: "pwwkew"
Output: 3
Explanation: The answer is "wke", with the length of 3. 
             Note that the answer must be a substring, "pwke" is a subsequence and not a substring.

找最大不重复子串,思路是：对字符串进行遍历，把每个字符串都放入一个vector<int>中

提前创建256个空间初始化为-1

因为每一个字母都是一个asiical码，可以把每个索引都放入字母Asiical对应的位置

记录最大不重复子串的开始位置。当当前循环的字母所在位置上已经有字母时，更新起始位置为重复的字母的索引

更新最大子串长度

代码实现如下：

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        vector<int> dict(256, -1);
        int maxLen = 0, start = -1;
        for (int i = 0; i != s.length(); i++) {
            if (dict[s[i]] > start)
                start = dict[s[i]];
            dict[s[i]] = i;
            maxLen = max(maxLen, i - start);
        }
        return maxLen;
    
    }
};

4. Median of Two Sorted Arrays

There are two sorted arrays nums1 and nums2 of size m and n respectively.

Find the median of the two sorted arrays. The overall run time complexity should be O(log (m+n)).

You may assume nums1 and nums2 cannot be both empty.

Example 1:

nums1 = [1, 3]
nums2 = [2]

The median is 2.0

Example 2:

nums1 = [1, 2]
nums2 = [3, 4]

The median is (2 + 3)/2 = 2.5

对两个已经排序的数组，寻找其中位数

第一个想到的算法就是对其进行归并排序，然后直接取数字。

看到网上还有一个简单的算法是用二分法

可以参考

https://www.cnblogs.com/ganganloveu/p/4180523.html

5. Longest Palindromic Substring

Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.

Example 1:

Input: "babad"
Output: "bab"
Note: "aba" is also a valid answer.

Example 2:

Input: "cbbd"
Output: "bb"

回文算法，暴力算法：直接遍历，然后向两边展开，看是不是回文，不过要分奇偶情况

还有一种简单算法，马拉车算法可以参考

https://www.cnblogs.com/zyf0163/p/4780174.html