这样的字符转换的dp挺经典的，

class Solution {

public:

    int minDistance(string word1, string word2) {

        int dp[word1.size()+1][word2.size()+1];

        for(int i = 0; i < word1.size()+1; i++)

          dp[i][0] = i;

        for(int i = 0; i < word2.size()+1; i++)

          dp[0][i] = i;

        for(int i = 0; i < word1.size(); i++) {

          for(int j = 0; j < word2.size(); j++) {

            if(word1[i] == word2[j])

              dp[i+1][j+1] = dp[i][j];

            else

              dp[i+1][j+1] = min(min(dp[i][j],dp[i][j+1]),dp[i+1][j])+1;

          }

        }

      return dp[word1.size()][word2.size()];

    }

};