151. Reverse Words in a String

public class Solution {
    public String reverseWords(String s) {
        char[] arr=s.toCharArray();
        reverse(arr, 0, arr.length-1);
        int i=0;
        int j=0;
        while(j<=arr.length)
        {
            if(j==arr.length||arr[j]==' ')
            {
                reverse(arr, i,j-1);
                j++;
                i=j;
            }
            else
                j++;
        }
        i=0;
        j=0;
        while(j<arr.length)
        {
            if(arr[j]==' '&&(j==0||arr[j-1]==' '))
                j++;
            else
            {
                arr[i]=arr[j];
                j++;
                i++;
            }
        }
        if(i>0&&i<=arr.length&&arr[i-1]==' ')
            i--;
        return new String(arr).substring(0,i);
    }
    private void reverse(char[] arr, int i, int j){
        while(i<j)
        {
            char t=arr[i];
            arr[i]=arr[j];
            arr[j]=t;
            i++;
            j--;
        }
    }
}