"String Chains" brain teaser

StringChains.java

package io.schmohl;

import java.util.*;

public class StringChain {

    // we can use depth first search for each word to calculate the length of the chain.

    static int longestChain(String[] words) {

        // create something with O(1) searching of words.
        final Set<String> wordMap = new HashSet<>();
        for (String e : words) wordMap.add(e);

        int max = -1;
        for (String target : words) {
            int k = depthFirstCount(target, wordMap);
            if (k > max) max = k;
        }

        return max;
    }

    private static int depthFirstCount(String target, Set<String> wordMap) {
        int n = target.length();
        int max = 0;

        if (!wordMap.contains(target)) return 0;

        for (int i = 0; i < n; i++) {
            String current = removeCharAt(target, i);
            // increase our depth by 1;
            int local = 1 + depthFirstCount(current, wordMap);
            // find the max depth recursively.
            if (local > max)
                max = local;
        }
        return max;
    }

    static String removeCharAt(String s, int i) {
        StringBuilder buf = new StringBuilder(s);
        buf.delete(i, i+1);
        return buf.toString();
    }
}