I need clarification of Java while loop conditional statement, when trying to find longest common prefix from an array of Strings

Question

I'm working on a problem - Longest Common Prefix - using Java, for this problem I must return the longest common prefix when given an array of Strings, for example an array of ["ale", "Altamonte", "alligator"] would return the Longest Common Prefix "al". One of the answers given for this problem was provided and I have posted it below, however I do not understand the conditional statement within the while loop, which is while (strs[i].indexOf(prefix) != 0). When I try to understand this conditional statement I perceive it as if parameter strs at index i, which can only be 1 through strs.length -1 , why put .indexOf(prefix) != 0 ? when I do a print out statement of System.out.println(strs[i].indexOf(prefix)); I get value of -1. I perceive this as some way for the loop to iterate backwards from strs[i] until the method finally gets to the longest common prefix? I also see that the variable prefix is equal to strs[0] so is the while loop conditional saying that since strs[i] cannot go to 0 then we get value of -1? I hope this question makes sense, I'm just struggling to understand this conditional statement and if anyone could help me understand how this code below is actually working I would very much appreciate the knowledge! many sincere thanks for any help given!

  public String longestCommonPrefix(String[] strs) {
        if (strs.length == 0) return "";
        String prefix = strs[0];
        for (int i = 1; i < strs.length; i++)
            while (strs[i].indexOf(prefix) != 0) {
 // System.out.println("Confused by this--> " + strs[i].indexOf(prefix));// this prints out -1 and im not sure why..
                prefix = prefix.substring(0, prefix.length() - 1);
                if (prefix.isEmpty()) return "";
            }        
        return prefix;
    }

Answer 1

// System.out.println("Confused by this--> " + strs[i].indexOf(prefix));// this prints out -1 and im not sure why..

-1 means that the prefix was not a prefix of the current string you are comparing it with. For example, foo is not a prefix of bar, so when you try to call "foo".indexOf("bar") you will indeed get '-1' because an index does not exist where this is true.

however I do not understand the conditional statement within the while loop, which is while (strs[i].indexOf(prefix) != 0)

All this condition is doing is checking if prefix is a prefix of the current string you comparing it with. If it returns 0 it means that it is, else -1 is returned and the while loop will continue to trim the last character of prefix until the condition is satisfied. If prefix is completely trimmed then return an empty string.

Example 1:

String[] s = {"al", "alamonte", "alligator"} 
prefix = s[0] // "al"

• "al" exist in "alimonte" -- "alimonte".indexOf("al") = 0
• "al" exist in "alligator" -- "alligator.indexOf("al") = 0
• return "al"
• This is a bad example to look into because "al" is conveniently at the beginning of the array and it fits into all other elements. The while loop is never entered, so we never see it in action.

Let's try another example:

String[] s = {"leet","leetcode", "leet", "leed"};
prefix = "leet"

• "leet" exist in "leetcode" -- leetcode.indexOf("leet") = 0
• "leet" exist in "leet" -- leet.indexOf("leet") = 0
• "leet" DOES NOT exist in "leed" -- leet.indexOf("leed") = -1
  • Enter while body and continue to trim last character.
  • trim 't' from prefix: "leet"
  • prefix = "lee"
  • check while condition again -> "lee" exist in "leed" -"leed".indexOf("lee") = 0
• We reached the end of our array and return prefix : "lee".

I took the code you provided and modified it with print statements, so that you can see how prefix is trimmed when index.Of() returns -1.

public class LongestCommonPrefix {

    public static String longestCommonPrefix(String[] strs) {
        if (strs.length == 0) return "";
        String prefix = strs[0];
        
        for (int i = 1; i < strs.length; i++) {
            System.out.println(strs[i] + ".indexOf(" + prefix + ")" + " =  " + strs[i].indexOf(prefix));
            while (strs[i].indexOf(prefix) != 0) {
                System.out.println("Trim the last character: " + prefix.charAt(prefix.length() - 1));
                prefix = prefix.substring(0, prefix.length() - 1);
                if (prefix.isEmpty())  {
                    System.out.println("No LCP exist after trimming all characters from prefix!");
                    return "";
                }
            }
            System.out.println("prefix after comparing with " + strs[i] + ": " + prefix + "\n");
        }
        return prefix;
    }
    
    public static void main(String[] args) {
        String[] s = {"leet","leetcode", "leet", "leed"};
        System.out.println("Input array: " + Arrays.toString(s) + "\n");
        System.out.println("LCP: " + longestCommonPrefix(s));
    }
}

Hope this helps out and best of luck!