題目：
Given two words word1 and word2, find the minimum number of steps required to convert word1 to word2. (each operation is counted as 1 step.)
You have the following 3 operations permitted on a word:
a) Insert a characterb) Delete a character
c) Replace a character
題解：
處理這道題也是用動態規劃。
動態陣列dp[word1.length+1][word2.length+1]
dp[i][j]表示從word1前i個字元轉換到word2前j個字元最少的步驟數。
假設word1現在遍歷到字元x，word2遍歷到字元y（word1當前遍歷到的長度為i，word2為j）。
以下兩種可能性：
1. x==y，那麼不用做任何編輯操作，所以dp[i][j] = dp[i-1][j-1]
2. x != y
(1) 在word1插入y， 那麼dp[i][j] = dp[i][j-1] + 1
(2) 在word1刪除x， 那麼dp[i][j] = dp[i-1][j] + 1
(3) 把word1中的x用y來替換，那麼dp[i][j] = dp[i-1][j-1] + 1
最少的步驟就是取這三個中的最小值。
最後返回 dp[word1.length+1][word2.length+1] 即可。
程式碼如下：
2 int len1 = word1.length();
3 int len2 = word2.length();
4
5 // len1+1, len2+1, because finally return dp[len1][len2]
6 int[][] dp = new int[len1 + 1][len2 + 1];
7
8 for (int i = 0; i <= len1; i++)
9 dp[i][0] = i;
10
11 for (int j = 0; j <= len2; j++)
12 dp[0][j] = j;
13
14
15 //iterate though, and check last char
16 for (int i = 1; i <= len1; i++) {
17 char c1 = word1.charAt(i-1);
18 for (int j = 1; j <= len2; j++) {
19 char c2 = word2.charAt(j-1);
20
21 //if last two chars equal
22 if (c1 == c2) {
23 //update dp value for +1 length
24 dp[i][j] = dp[i-1][j-1];
25 } else {
26 int replace = dp[i-1][j-1] + 1;
27 int insert = dp[i-1][j] + 1;
28 int delete = dp[i][j-1] + 1;
29
30 int min = Math.min(replace, insert);
31 min = Math.min(min,delete);
32 dp[i][j] = min;
33 }
34 }
35 }
36
37 return dp[len1][len2];
38 }
Reference：
http://www.programcreek.com/2013/12/edit-distance-in-java/
http://blog.csdn.net/linhuanmars/article/details/24213795