[LeetCode] 344. Reverse String

Write a function that takes a string as input and returns the string reversed.

Example:
Given s = "hello", return "olleh".

Solution 1:

Approach: use StringBuilder.reverse() method

public class Solution {
    public String reverseString(String s) {
        StringBuilder sb=new StringBuilder(s);
        return sb.reverse().toString();
    }
}

Solution 2:

public class Solution {
    public String reverseString(String s) {
        StringBuilder sb=new StringBuilder();
        for(int i=s.length()-1; i>=0; i--){
            sb.append(s.charAt(i));
        }
        return sb.toString();
    }
}

[LeetCode] 3. Longest Substring Without Repeating Characters

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

Examples:

Given "abcabcbb", the answer is "abc", which the length is 3.

Given "bbbbb", the answer is "b", with the length of 1.

Given "pwwkew", 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.

Solution 1: 

Approach: brute force

Time Complexity: O(n^2)

Space Complexity: O(n)

public class Solution {
    public int lengthOfLongestSubstring(String s) {
        if(s.length()==0||s==null) return 0;
        boolean[] exist=new boolean[256];
        int max=0;
        int start=0;
        for(int i=0; i<s.length(); i++){
            if(exist[s.charAt(i)]){
                for(int j=start; j<i; j++){
                    if(s.charAt(j)==s.charAt(i)){
                        start=j+1;
                        break;
                    }
                    exist[s.charAt(j)]=false;
                }
            }else{
                exist[s.charAt(i)]=true;
                max=Math.max(max, i-start+1);
            }
        }
        return max;
    }
}

Solution 2:
Approach: Hash Table
Time complexity: O(n)
Space complexity: O(n)

public class Solution {
    public int lengthOfLongestSubstring(String s) {
        if(s.length()==0||s==null) return 0;
        HashMap<Character, Integer> hashMap=new HashMap<Character, Integer>();
        int max=0;
        int start=0;
        for(int i=0; i<s.length(); i++){
            if(hashMap.containsKey(s.charAt(i))){
                max=Math.max(max, hashMap.size());
                while(hashMap.containsKey(s.charAt(i))){
                    hashMap.remove(s.charAt(start));
                    start++;
                }
                hashMap.put(s.charAt(i), i);
            }else{
                hashMap.put(s.charAt(i), i);
                max=Math.max(max, hashMap.size());
            }
        }
        return max;  
    }
}

[LeetCode] 2. Add Two Numbers

You are given two linked lists representing two non-negative numbers. 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.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

Time Complexity: O(max(m,n))

Space Complexity: O(max(m,n))

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummy=new ListNode(0);
        ListNode tail=dummy;
        int carry=0;
        if(l1==null) return l2;
        if(l2==null) return l1;
        
        while(l1!=null||l2!=null){
            if(l1!=null){
                carry+=l1.val;
                l1=l1.next;
            }
            if(l2!=null){
                carry+=l2.val;
                l2=l2.next;
            }
            tail.next=new ListNode(carry%10);
            tail=tail.next;
            carry/=10;
        }
        if(carry==1){
            tail.next=new ListNode(1);
        }
        return dummy.next;
    }
}

Helpful Job Hunting Sites

北美IT求职有用的网站总结

Coding practice sites

• LeetCode OJ: https://leetcode.com
• LintCode: http://www.lintcode.com/en/
• HackerRank: https://www.hackerrank.com
• Topcoder: https://www.topcoder.com

Technical interview questions sites

• Cracking the Coding Interview: amazon-book
• ProgramCreek: http://www.programcreek.com
• GeeksforGeeks: http://www.geeksforgeeks.org

• System Design: http://www.hiredintech.com/system-design/
• Design Pattern: http://www.programcreek.com/java-design-patterns-in-stories/

• Java concurrency: http://tutorials.jenkov.com/java-concurrency/index.html
• Java basic: http://www.java2novice.com/java-interview-questions/
• Java basic2: http://www.javatpoint.com/corejava-interview-questions

• Java: http://www.tutorialspoint.com/java/
• Android: http://www.tutorialspoint.com/android/
• Python: http://www.tutorialspoint.com/python/
• C: http://www.tutorialspoint.com/cprogramming/
• Design Pattern: http://www.tutorialspoint.com/design_pattern/design_pattern_overview.htm

Interview reviews sites

• Glassdoor: https://www.glassdoor.com/index.htm
• CareerCup: https://www.careercup.com/page
• GeeksforGeeks: http://www.geeksforgeeks.org/about/interview-corner/
• (Chinese) 1亩3分地: http://www.1point3acres.com/bbs/
• (Chinese) 米群网: http://www.meetqun.com/forum.php
• (Korean) WorkingUS: http://www.workingus.com/v3/forums/forum/forum/interview/

Resume apply sites

• LinkedIn: https://www.linkedin.com
• Glassdoor: https://www.glassdoor.com/index.htm
• Indeed: http://www.indeed.com
• Monster: http://www.monster.com
• Dice: http://www.dice.com
• CyberCoders: https://www.cybercoders.com

Startup company jobs posting sites

• StartUpHire: http://www.startuphire.com
• AngelList: https://angel.co

Code share sites

• GitHub: https://github.com
• Bitbucket: https://bitbucket.org

Online compiler sites

• ideone: ideone.com
• codepad: codepad.org

Online course sites

• Coursera: https://www.coursera.org
• Udacity: https://www.udacity.com
• edX: https://www.edx.org
• Treehouse: https://teamtreehouse.com/home

Other helpful sites

• Stack Overflow: http://stackoverflow.com

[LeetCode] 1. Two Sum

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.

Example:

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

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

Solution 1:
Approach: Brute force
Time complexity: O(n^2)
Space complexity: O(1)

public class Solution {
    public int[] twoSum(int[] nums, int target) {
    
        int[] result=new int[2];
        if(nums==null||nums.length==0) return result;
        for(int i=0; i<nums.length; i++){
            for(int j=i+1; j<nums.length; j++){
                if(nums[i]+nums[j]==target){
                    result[0]=i;
                    result[1]=j;
                }
            }
        }
        return result;       
    }
}

Solution 2:
Approach: Hash Table
Time Complexity: O(n)
Space Complexity: O(n)

public class Solution {
    public int[] twoSum(int[] nums, int target) {
       
        int[] result=new int[2];
        if(nums==null||nums.length==0) return result;
        HashMap<Integer, Integer> hashMap=new HashMap<Integer, Integer>();
        for(int i=0; i<nums.length; i++){
            if(hashMap.containsKey(target-nums[i])){
                result[0]=i;
                result[1]=hashMap.get(target-nums[i]);
            }else{
                hashMap.put(nums[i], i);
            }
        }
        return result;
    }
}

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