LongestCommonPrefix.java

/**
 * Write a function to find the longest common prefix string amongst an array
 * of strings.
 * 
 * Tags: String
 */
class LongestCommonPrefix {
    
    public static void main(String[] args) {
        
    }
    
    /**
     * Find common prefix one by one from the end of the input string array
     * Overwrite the ith string with common prefix result
     * Thus space usage is reduced
     * Return first in group
     */
    public static String longestCommonPrefix(String[] strs) {
        for (int i = strs.length - 2; i >= 0 ; i--) {
            strs[i] = commonPrefix(strs[i + 1], strs[i]);
        }
        return strs[0];
    }
    
    /**
     * Get length of two strings
     * Loop over each char till one length runs out
     * If same char, append it to result
     * If not same, break
     * Return result
     */
    private static String commonPrefix(String a, String b) {
        StringBuilder pref = new StringBuilder();
        int lenA = a.length();
        int lenB = b.length();
        int i = 0;
        while (i < lenA && i < lenB) {
            if (a.charAt(i) == b.charAt(i)) pref.append(a.charAt(i));
            else break;
            i++;
        }    
        return pref.toString();
    }
    
    /**
     * Only need to know the length of prefix
     * Initialize result with first word
     * Traverse from second word to last word
     * Get minimum length of current result and next word
     * Check whether prefix is that long in that length
     */
    public String longestCommonPrefix(String[] strs) {
        if (strs == null) return null;
        if (strs.length == 0) return "";
        if (strs.length == 1) return strs[0];

        String word = strs[0];
        int prefixLength = word.length();

        for (int i = 1; i < strs.length; ++i) {
            String nextWord = strs[i];
            prefixLength = Math.min(prefixLength, nextWord.length());
            for (int j = 0; j < prefixLength; ++j)
                if (word.charAt(j) != nextWord.charAt(j)) {
                    prefixLength = j;
                    break;
                }
        }

        return word.substring(0, prefixLength);
    }
}