LeetCode_14. Longest Common Prefix

14. Longest Common Prefix

Easy

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.

package leetcode.easy;

public class LongestCommonPrefix {
	@org.junit.Test
	public void test() {
		String[] strs1 = { "flower", "flow", "flight" };
		String[] strs2 = { "dog", "racecar", "car" };
		System.out.println(longestCommonPrefix1(strs1));
		System.out.println(longestCommonPrefix1(strs2));
		System.out.println(longestCommonPrefix2(strs1));
		System.out.println(longestCommonPrefix2(strs2));
		System.out.println(longestCommonPrefix3(strs1));
		System.out.println(longestCommonPrefix3(strs2));
		System.out.println(longestCommonPrefix4(strs1));
		System.out.println(longestCommonPrefix4(strs2));
	}

	public String longestCommonPrefix1(String[] strs) {
		if (strs.length == 0) {
			return "";
		}
		String prefix = strs[0];
		for (int i = 1; i < strs.length; i++) {
			while (strs[i].indexOf(prefix) != 0) {
				prefix = prefix.substring(0, prefix.length() - 1);
				if (prefix.isEmpty()) {
					return "";
				}
			}
		}
		return prefix;
	}

	public String longestCommonPrefix2(String[] strs) {
		if (strs == null || 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) {
					return strs[0].substring(0, i);
				}
			}
		}
		return strs[0];
	}

	public String longestCommonPrefix3(String[] strs) {
		if (strs == null || strs.length == 0) {
			return "";
		}
		return longestCommonPrefix3(strs, 0, strs.length - 1);
	}

	private String longestCommonPrefix3(String[] strs, int l, int r) {
		if (l == r) {
			return strs[l];
		} else {
			int mid = (l + r) / 2;
			String lcpLeft = longestCommonPrefix3(strs, l, mid);
			String lcpRight = longestCommonPrefix3(strs, mid + 1, r);
			return commonPrefix(lcpLeft, lcpRight);
		}
	}

	String commonPrefix(String left, String right) {
		int min = Math.min(left.length(), right.length());
		for (int i = 0; i < min; i++) {
			if (left.charAt(i) != right.charAt(i)) {
				return left.substring(0, i);
			}
		}
		return left.substring(0, min);
	}

	public String longestCommonPrefix4(String[] strs) {
		if (strs == null || strs.length == 0) {
			return "";
		}
		int minLen = Integer.MAX_VALUE;
		for (String str : strs) {
			minLen = Math.min(minLen, str.length());
		}
		int low = 1;
		int high = minLen;
		while (low <= high) {
			int middle = (low + high) / 2;
			if (isCommonPrefix(strs, middle)) {
				low = middle + 1;
			} else {
				high = middle - 1;
			}
		}
		return strs[0].substring(0, (low + high) / 2);
	}

	private boolean isCommonPrefix(String[] strs, int len) {
		String str1 = strs[0].substring(0, len);
		for (int i = 1; i < strs.length; i++) {
			if (!strs[i].startsWith(str1)) {
				return false;
			}
		}
		return true;
	}
}