14. Longest Common Prefix

class Solution {
    public String longestCommonPrefix(String[] strs) {
        if(strs.length==0)
            return "";
        if(strs.length==1)
            return strs[0];
        return find(strs, strs[0],1);
    }
    private String find(String[] strs, String prefix, int pos){
        if(pos==strs.length)
            return prefix;
        if(prefix.equals("")||strs[pos].equals("")){
            return "";
        }
        int ed = Math.min(strs[pos].length(),prefix.length()); 
        for(int i=0;i<ed;i++){
            if(strs[pos].charAt(i)!=prefix.charAt(i)){
                return find(strs, prefix.substring(0,i),pos+1);
            }
        }
        return find(strs, prefix.substring(0,ed),pos+1);
    }
}

使用java.lang.String.indexOf(String)方法(更快)

class Solution {
    public String longestCommonPrefix(String[] strs) {
        if(strs==null || strs.length==0) return "";
        String prefix = strs[0];
        int i=1;
        while(i<strs.length){
            while(strs[i].indexOf(prefix)!=0)
                prefix = prefix.substring(0,prefix.length()-1);
            i++;
        }
        return prefix;
    }
}

贴出java.lang.String.indexOf(String)最终实现

/**
     * Code shared by String and StringBuffer to do searches. The
     * source is the character array being searched, and the target
     * is the string being searched for.
     *
     * @param   source       the characters being searched.
     * @param   sourceOffset offset of the source string.
     * @param   sourceCount  count of the source string.
     * @param   target       the characters being searched for.
     * @param   targetOffset offset of the target string.
     * @param   targetCount  count of the target string.
     * @param   fromIndex    the index to begin searching from.
     */
    static int indexOf(char[] source, int sourceOffset, int sourceCount,
            char[] target, int targetOffset, int targetCount,
            int fromIndex) {
        if (fromIndex >= sourceCount) {
            return (targetCount == 0 ? sourceCount : -1);
        }
        if (fromIndex < 0) {
            fromIndex = 0;
        }
        if (targetCount == 0) {
            return fromIndex;
        }

        char first = target[targetOffset];
        int max = sourceOffset + (sourceCount - targetCount);

        for (int i = sourceOffset + fromIndex; i <= max; i++) {
            /* Look for first character. */
            if (source[i] != first) {
                while (++i <= max && source[i] != first);
            }

            /* Found first character, now look at the rest of v2 */
            if (i <= max) {
                int j = i + 1;
                int end = j + targetCount - 1;
                for (int k = targetOffset + 1; j < end && source[j]
                        == target[k]; j++, k++);

                if (j == end) {
                    /* Found whole string. */
                    return i - sourceOffset;
                }
            }
        }
        return -1;
    }