#N922. Sort Array By Parity II
problem link

#solution

class Solution {
    public int[] sortArrayByParityII(int[] nums) {
        int[] ans=new int[nums.length];
        int evenIndex=0, oddIndex=1;
        
        for(int i=0; i<nums.length; i++){
            if(nums[i]%2==0){
                ans[evenIndex]=nums[i];
                evenIndex+=2;
            }
            else{
                ans[oddIndex]=nums[i];
                oddIndex+=2;
            }
        }
        
        return ans;
    }
}

#N1441. Build an Array With Stack Operations

problem link

#solution

class Solution {
    public List<String> buildArray(int[] target, int n) {
        ArrayList<String> stack=new ArrayList<String>();
        int x=0;
        for(int i=1;i<=n && x<target.length; i++){
            stack.add("Push");
            if(target[x]==i) x++;
            else stack.add("Pop");
        }
        
        return stack;
    }
}

#N1108. Defanging an IP Address
problem link

#solution

class Solution {
    public String defangIPaddr(String address) {
        String res="";
        for(char ch:address.toCharArray()){
            if(ch=='.')
                res+="[.]";
            else
                res+=ch;
        }
        
        return res;
    }
}

#N771. Jewels and Stones
problem link

#solution

class Solution {
    public int numJewelsInStones(String jewels, String stones) {
        int count=0;
        for(char j:jewels.toCharArray()){
            for(char s:stones.toCharArray()){
                if(j==s) count++;
            }
        }
        
        return count;
    }
}

#N1678. Goal Parser Interpretation
problem link

#solution

class Solution {
    public String interpret(String command) {
        String res="";
        for(int i=0; i<command.length()-1; i++){
            if(command.charAt(i)=='('&&command.charAt(i+1)==')')
                res+="o";
            else if(command.charAt(i)=='('&&command.charAt(i+1)=='a')
                res+="al";
            else if(command.charAt(i)=='G')
                res+="G";
        }
        if(command.charAt(command.length()-1)=='G') res+="G";
        
        return res;
    }
}

#N1221. Split a String in Balanced Strings
problem link

#solution

class Solution {
    public int balancedStringSplit(String s) {
        int count=0, balance=0;
        for(char ch:s.toCharArray()){
            if(ch=='R')
                balance--;
            if(ch=='L')
                balance++;
            if(balance==0)
                count++;
        }
        
        return count;
    }
}

#N1859. Sorting the Sentence
problem link

#solution

class Solution {
    public String sortSentence(String s) {
        String[] words=s.split(" ");
        String[] sorted=new String[words.length];
        String res="";
        
        for(String word:words){
            sorted[Character.getNumericValue(word.charAt(word.length()-1))-1]=word.substring(0, word.length()-1);
        }
        
        for(int i=0; i<sorted.length; i++){
            res+=sorted[i];
            if(i!=sorted.length-1)
                res+=" ";
        }
        
        return res;
    }
}

#N1614. Maximum Nesting Depth of the Parentheses
problem link

#solution

class Solution {
    public int maxDepth(String s) {
        int count=0, depth=0;
        for(char ch:s.toCharArray()){
            if(ch=='(')
                count++;
            if(ch==')'){
                depth=Math.max(depth, count--);
            }
        }
        
        return depth;
    }
}

#N1832. Check if the Sentence Is Pangram
problem link

#solution

class Solution {
    public boolean checkIfPangram(String sentence) {
        int[] alphabet=new int[26];
        
        for(char ch:sentence.toCharArray()){
            alphabet[ch-'a']++;
        }
        
        for(int num:alphabet)
            if(num==0)
                return false;
        
        return true;
    }
}

#statistics
Till this time, 91 in total,
86 easy;
5 medium;
type of problem solutions have been posted

#N709. To Lower Case
problem link

#solution

class Solution {
    public String toLowerCase(String s) {
        char[] res=s.toCharArray();
        
        for(int i=0; i<res.length; i++){
            if(res[i]>='A'&&res[i]<='Z')
                res[i]=(char)(res[i]+32);
        }
        
        return String.valueOf(res);
    }
}

or another option, using the String method toLowerCase(), just write return s.toLowerCase(); .

#N1844. Replace All Digits with Characters
problem link

#solution

class Solution {
    public String replaceDigits(String s) {
        char[] ch=s.toCharArray();
        
        for(int i=1; i<ch.length; i=i+2){
            ch[i]=(char)(ch[i-1]+Character.getNumericValue(ch[i]));
        }
        
        return String.valueOf(ch);
    }
}

#N2000. Reverse Prefix of Word
problem link

#solution

class Solution {
    public String reversePrefix(String word, char ch) {
        char[] letters=word.toCharArray();
        int index=-1;
        char temp;
        for(int i=0; i<letters.length; i++){
            if(letters[i]==ch){
                index=i;
                for(int j=0; j<(index+1)/2; j++){
                    temp=letters[j];
                    letters[j]=letters[index-j];
                    letters[index-j]=temp;
                }
                break;
            }
        }
        
        return String.valueOf(letters);
    }
}

#N1704. Determine if String Halves Are Alike
problem link

#solution

class Solution {
    char[] vowels={'a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U'};
    public boolean halvesAreAlike(String s) {
        String half1=s.substring(0, s.length()/2);
        String half2=s.substring(s.length()/2);
        int count=0;
        
        for(int i=0; i<half1.length(); i++){
            if(isVowel(half1.charAt(i)))
                count++;
            if(isVowel(half2.charAt(i)))
                count--;
        }
        
        return count==0;
    }
    
    boolean isVowel(char ch){
        for(char vowel:vowels){
            if(ch==vowel)
                return true;
        }
        return false;
    }
}

#N1967. Number of Strings That Appear as Substrings in Word
problem link

#solution

class Solution {
    public int numOfStrings(String[] patterns, String word) {
        int count=0;
        for(String pattern:patterns){
            if(word.contains(pattern))
                count++;
        }
        
        return count;
    }
}

#N1309. Decrypt String from Alphabet to Integer Mapping
problem link

#solution

class Solution {
    public String freqAlphabets(String s) {
        StringBuilder res=new StringBuilder();
        for(int i=0; i<s.length(); i++){
            if(i<s.length()-2&&s.charAt(i+2)=='#'){
                res.append((char)(Integer.parseInt(s.substring(i, i+2))+96));
                i=i+2;
            }
            else
                res.append((char)(Character.getNumericValue(s.charAt(i))+96));
        }
        
        return res.toString();
    }
}

#useful
Till this time, we used only Strings to add Strings each other, but we got the point that this is not efficient at all. So then, we will use the StringBuilder class as they are mutable unlike String. And they are more efficient in adding and reversing Strings.
For more info: https://docs.oracle.com/javase/9/docs/api/java/lang/StringBuilder.html

#medium

#N1769. Minimum Number of Operations to Move All Balls to Each Box
problem link

#solution

class Solution {
    public int[] minOperations(String boxes) {
        char[] ch=boxes.toCharArray();
        int[] answer=new int[boxes.length()];
        int count;
        
        for(int i=0; i<answer.length; i++){
            count=0;
            for(int j=0; j<answer.length; j++)
                if(ch[j]=='1')
                    count+=Math.abs(i-j);
                
            answer[i]=count;
        }
        
        return answer;
    }
}

#medium

#N1689. Partitioning Into Minimum Number Of Deci-Binary Numbers
problem link

#solution

class Solution {
    public int minPartitions(String n) {
        int max=0;
        
        for(char num:n.toCharArray())
            max=Math.max(max, num-'0');
        
        return max;
    }
}

#N944. Delete Columns to Make Sorted
problem link

#solution

class Solution {
    public int minDeletionSize(String[] strs) {
        int count=0;
        for(int i=0; i<strs[0].length(); i++){
            for(int j=0; j<strs.length-1; j++){
                if(strs[j].charAt(i)>strs[j+1].charAt(i)){
                    count++;
                    break;
                }
            }
        }
        
        return count;
    }
}