[leetcode]14. Longest Common Prefix 最长公共前缀

Write a function to find the longest common prefix string amongst an array of strings.

If there is no common prefix, return an empty string "".

Example 1:

Input: ["flower","flow","flight"]
Output: "fl"

Example 2:

Input: ["dog","racecar","car"]
Output: ""
Explanation: There is no common prefix among the input strings.

Note:

All given inputs are in lowercase letters a-z.

Solution1:  

Compare characters from top to bottom on the same column (same character index of the strings) before moving on to the next column.

code

/*
Time complexity : O(S), where S is the sum of all characters in all strings. 
Space complexity : O(1). We only used constant extra space. 
*/
class Solution {
    public String longestCommonPrefix(String[] strs) {
        if (strs.length == 0)  return "";
        for (int i = 0; i < strs[0].length(); i++) {
            char c = strs[0].charAt(i);
            for (int j = 1; j < strs.length; j++) {
                if (i == strs[j].length() || strs[j].charAt(i) != c) { // 除str[0]外的一个String被扫完了
                    return strs[0].substring(0, i);
                }
            }
        }
        // 有可能strs[0]扫完了都没碰到垂直方向不相同的字符
        return strs[0];
    }
}