text: class Solution:
      """
      @param word1: A string
      @param word2: A string
      @return: The minimum number of steps.
      """
      def minDistance(self, word1, word2):
          # write your code here
          if not word1:
              return len(word2)
              
          if not word2:
              return len(word1)
              
          n, m = len(word1), len(word2)
          
          dp = [[0] * (m + 1) for _ in range(n + 1)]
          
          for i in range(n + 1):
              dp[i][0] = i
          for j in range(m + 1):
              dp[0][j] = j
              
          for i in range(1, n + 1):
              for j in range(1, m + 1):
                  dp[i][j] = min(dp[i - 1][j], dp[i][j - 1]) + 1
                  
                  if word1[i - 1] == word2[j - 1]:
                      dp[i][j] = min(dp[i][j], dp[i - 1][j - 1])
                  else:
                      dp[i][j] = min(dp[i][j], dp[i - 1][j - 1] + 1)
                      
          return dp[n][m]