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Problem Statement

You have given a String (containing multiple words), reverse the string word by word.

Example 

Input :- "the sky is blue"
Output :- "blue is sky the"

Input :- "  hello world!  "
Output :- "world! hello"

Input :- "sun rise in the morning"
Output :- "morning the in rise sun"

Solution

This problem can be solved in following steps :-

1. If the input string is null or empty, then return the same string as an output otherwise moves to the next step.
2. Create an array of characters from the input string and an object of StringBuffer class for writing output.
3. Traverse the character array from the end and find out the end and start index of a single word.
4. Write the word found in the previous step followed by a single space.
5. Repeat steps 3 and 4, until the array is completely traversed.

Program

public class Main {

	public static void main(String[] args) {		
		Main main = new Main();
		String result = main.reverseWords("sun rise in the morning");
		System.out.print(result);
	}
	
	/* Solution */
	public String reverseWords(String str) {
        
        if(str == null || str.length() == 0) return str;
        
        StringBuilder sb = new StringBuilder();
        
        char[] array = str.toCharArray();        
        int end = -1, i;
        
        for(i=array.length-1; i>=0; i--){
            
            if(array[i] == ' '){
                if(end != -1){
                    write(array, i+1, end, sb); 
                    end = -1;
                }                
            }else {
                if(end == -1) end = i;
            }
        }
        if(end != -1){
            write(array, 0, end, sb); 
        }
        
        return sb.length() > 0 ? sb.substring(0, sb.length()-1).toString() : "";
    }    
    
    public void write(char[] array, int start, int end, StringBuilder sb){        
        while(start<=end){     
            sb.append(String.valueOf(array[start++]));
        }
        sb.append(" ");
    }
}

Result

morning the in rise sun

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2. Reverse Vowels of a String
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