class Solution {public:    int minDistance(string word1, string word2) {        int ret = 0;        size_t len1 = word1.length();        size_t len2 = word2.length();        if (len1 * len2 == 0) return abs((int)len1 - (int)len2);        //    dp[i][j] = min steps for word1[0..i-1] to word2[0..j-1]                //        word1[i-1] == word2[j-1]?        //            dp[i][j] = dp[i-1][j-1]        //        not equal        //            dp[i][j] = min(dp[i-1][j-1], dp[i-1][j], dp[i][j-1]) + 1        //        dp[len1][len2]        vector<vector<int>> dp;        dp.resize(len1 + 1);        for (int i = 0; i <= len1; i++)        {            dp[i].resize(len2 + 1);            std::fill(dp[i].begin(), dp[i].end(), 0);        }        for (int i = 1; i <= len1; i++) dp[i][0] = i;        for (int j = 1; j <= len2; j++) dp[0][j] = j;                //    Go        for (int i = 1; i <= len1; i ++)        for (int j = 1; j <= len2; j++)        {            if (word1[i - 1] == word2[j - 1])            {                dp[i][j] = dp[i - 1][j - 1];            }            else            {                int v1 = dp[i - 1][j - 1];                int v2 = dp[i - 1][j];                int v3 = dp[i][j - 1];                dp[i][j] = std::min(std::min(v1, v2), v3) + 1;            }        }        return dp[len1][len2];    }};