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/**
 * Write a function to find the longest common prefix string amongst an array of strings.
 */
 
public class Solution {
    public String longestCommonPrefix(String[] strs) {
        if(strs.length == 0)
            return "";
        int n = strs.length;
        for(int i = 0; i < strs[0].length(); i++){
            for(int j = 1; j < n; j++){
                // pay attention to the sequence, or it will throw out the runtime error.
                // because the charAt(i) function could not be checked before the length of the string j is guaranteed. 
                if(strs[j].length() <= i || strs[j].charAt(i) != strs[0].charAt(i)){
                    return strs[0].substring(0, i);
                }
            }
        }
        return strs[0];
    }
}