14. Longest Common Prefix 

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class Solution {
    public String longestCommonPrefix(String[] strs) {
        String sentinel = strs[0];

        int endIndex = -1; // inclusive
        for (int i = 0; i < sentinel.length(); i++) {
            for (int j = 0; j < strs.length; j++) {
                if (i >= strs[j].length() || strs[j].charAt(i) != sentinel.charAt(i)) {
                    return sentinel.substring(0, endIndex + 1);
                }
            }
            endIndex = i;
        }

        return sentinel.substring(0, endIndex + 1);
    }
}

References

14. Longest Common Prefix