Update 072._edit_distance.md

docs/Leetcode_Solutions/072._edit_distance.md

@@ -112,7 +112,8 @@ class Solution(object):
         """
         if len(word1) == 0 or len(word2) == 0:  # corner cases
             return max(len(word1), len(word2))
-        dp = [[i+j for j in range(len(word2)+1)] for i in range(len(word1)+1)]
+	# 这里第一行第一列初始值一定要为i+j，因为当另一个单词为空的时候很明显至少需要i或者j次edit
+        dp = [[i+j for j in range(len(word2)+1)] for i in range(len(word1)+1)] 
         for i in range(1, len(word1)+1):
             for j in range(1, len(word2)+1):
                 tmp_dist = 0 if word1[i-1] == word2[j-1] else 1