[LeetCode] 1002. Find Common Characters

Given an array A of strings made only from lowercase letters, return a list of all characters that show up in all strings within the list (including duplicates).  For example, if a character occurs 3 times in all strings but not 4 times, you need to include that character three times in the final answer.

You may return the answer in any order.

Example 1:

Input: ["bella","label","roller"]
Output: ["e","l","l"]

Example 2:

Input: ["cool","lock","cook"]
Output: ["c","o"]

Note:

1. 1 <= A.length <= 100
2. 1 <= A[i].length <= 100
3. A[i][j] is a lowercase letter

查找常用字符。

题意是给一个装了单词的数组，请求出这些单词里面共同的字母，以list形式输出。如果有多个相同的字母也需要多次输出。

思路是用hashmap记录input中第一个单词里面每个字母的出现次数，然后从第二个单词开始，也是去计算每个字母出现的次数，但是把某个字母在某个单词中出现的最少的次数加入结果集。举个例子，比如cool里面o出现了两次，但是在lock里面o只出现了一次，记录o的时候只记录最少的出现次数。最后遍历结果集里面每个单词，根据次数拼接成最后要输出的list。

时间O(n^2) - 每个单词 * 每个单词的长度

空间O(n)

class Solution {
    public List<String> commonChars(String[] A) {
        List<String> res = new ArrayList<>();
        int[] count = new int[26];
        Arrays.fill(count, Integer.MAX_VALUE);
        for (String str : A) {
            int[] cnt = new int[26];
            for (char c : str.toCharArray()) {
                cnt[c - 'a']++;
            }
            for (int i = 0; i < 26; i++) {
                count[i] = Math.min(count[i], cnt[i]);
            }
        }

        for (int i = 0; i < 26; i++) {
            while (count[i] > 0) {
                res.add("" + (char) (i + 'a'));
                count[i]--;
            }
        }
        return res;
    }
}

JavaScript实现

/**
 * @param {string[]} A
 * @return {string[]}
 */
var commonChars = function(A) {
    let arrayList = [];
    for (let str of A) {
        let arr = new Array(26).fill(0);
        for (let i = 0; i < str.length; i++) {
            arr[str[i].charCodeAt() - 'a'.charCodeAt()]++;
        }
        arrayList.push(arr);
    }
    let res = [];
    for (let i = 0; i < 26; i++) {
        let min = 100;
        for (let j = 0; j < arrayList.length; j++) {
            if (arrayList[j][i] < min) {
                min = arrayList[j][i];
            }
        }
        for (let k = 0; k < min; k++) {
            res.push(String.fromCharCode(i + 'a'.charCodeAt()));
        }
    }
    return res;
};

LeetCode 题目总结