js program to longest prefix in string

To write a JavaScript program that finds the longest common prefix among an array of strings, you can follow these steps. The program will take an array of strings as input and return the longest prefix that is common to all the strings in the array. If there is no common prefix, it will return an empty string.

Here’s a simple implementation of such a program using JavaScript:

function longestCommonPrefix(strs) {

    if (strs.length === 0) return ""; // Check if the array is empty

    let prefix = strs[0]; // Start with the first string in the array as the prefix

    for (let i = 1; i < strs.length; i++) {

        while (strs[i].indexOf(prefix) !== 0) { // Check if the prefix is not at the start of the string

            prefix = prefix.substring(0, prefix.length - 1); // Reduce the prefix by one character from the end

            if (prefix === "") return ""; // If prefix is empty, return it

        }

    }

    return prefix; // Return the found prefix

}

// Example usage:

const strings = ["flower", "flow", "flight"];

console.log(longestCommonPrefix(strings)); // Output: "fl"

How the Code Works:

1. **Initialization**: The `prefix` variable is initialized to the first string of the array.

2. **Iteration**: The program iterates over the rest of the strings in the array.

3. **Comparison**: Inside the loop, a `while` loop checks whether the current `prefix` is a prefix of the string at the current index. If it isn't, the last character of the `prefix` is removed.

4. **Result**: Once all strings are checked, the longest common prefix is returned. If no common prefix is found during the process, an empty string is returned.

This approach efficiently reduces the prefix size until it matches all strings or becomes empty.

Let's break down the JavaScript code for finding the longest common prefix among an array of strings in more detail. The algorithm is designed to iteratively refine a candidate prefix by comparing it with each string in the array.

Step-by-Step Explanation of the Code:

1. **Function Definition:**

function longestCommonPrefix(strs) {

The function `longestCommonPrefix` is defined with one parameter, strs, which is expected to be an array of strings.

2. **Initial Check:**

    if (strs.length === 0) return ""; // Check if the array is empty

The function first checks if the array is empty. If it is, there are no strings to compare, and the function immediately returns an empty string, indicating that there is no common prefix.

3. **Initialize the Prefix:**

    let prefix = strs[0]; // Start with the first string in the array as the prefix

The `prefix` is initially set to the first string in the array. This string will serve as the starting point for finding the common prefix. The assumption here is that the longest common prefix cannot be longer than any string in the array, so starting with a full string is a logical first step.

4. **Iterate Over the Array:**

    for (let i = 1; i < strs.length; i++) {

We start a loop from the second element of the array (index 1) to the end. The purpose of this loop is to compare the current `prefix` against each string in the array.

5. **Check and Reduce Prefix:**

        while (strs[i].indexOf(prefix) !== 0) {

Inside the loop, a `while` loop checks if the current `prefix` starts at the beginning of the string at index `i. The indexOf` method returns `0` if `prefix` is found at the start of the string. If it's not found at the start, it returns a positive value or -1.

            prefix = prefix.substring(0, prefix.length - 1);

If the `prefix` is not found at the start, the code reduces the `prefix` by removing the last character. This is done using the `substring` method, which creates a new string starting from the first character up to, but not including, the last character of the current prefix.

            if (prefix === "") return ""; // If prefix is empty, return it

After trimming the prefix, we check if it has been reduced to an empty string. If so, it means there is no common prefix among the strings, and the function returns an empty string.

6. **Return the Longest Common Prefix:**

    return prefix; // Return the found prefix

}

After completing the loop for all strings in the array, whatever remains of the `prefix` is the longest common prefix. It's then returned as the result.

7. **Example Usage:**

const strings = ["flower", "flow", "flight"];

console.log(longestCommonPrefix(strings)); // Output: "fl"

This example demonstrates how the function can be called. The output `"fl"` is the longest common prefix of the strings "flower", "flow", and "flight".

Summary:

This approach effectively reduces the search space with each non-matching character found, ensuring that the function only returns the longest common prefix present in all given strings. The algorithm performs well by reducing the candidate prefix length step by step, only scanning through each string until the common prefix is found or dismissed.