Merge pull request #149 from qilingit/day31

Add Java solution for day31: Edit Distance

Author: mincong-h

May-LeetCoding-Challenge/31-Edit-Distance/Edit-Distance.java

@@ -0,0 +1,45 @@
+/**
+ * Dynamic programming, operation of previous string plus one if the last character is not the same
+ * XXXb -> YYYc => operation(XXX to YYY) + 1
+ * XXXm -> XXXm => operation(XXX to YYY)
+ */
+class Solution {
+    public int minDistance(String word1, String word2) {
+        int m = word1.length(), n = word2.length();
+        int dp[][] = new int[m+1][n+1];
+
+        /**
+         * Construct a dp table
+         * __|''|a|b|c|
+         * ''| 0|1|2|3|...
+         * ''| 0
+         * d | 1
+         * e | 2
+         * f | 3
+         */
+        for (int i = 0; i <= m; i++) {
+            dp[i][0] = i;
+        }
+
+        for (int j = 0; j <= n; j++) {
+            dp[0][j] = j;
+        }
+
+        for (int i = 0; i < m; i++) {
+            for (int j = 0; j < n; j++) {
+                char c1 = word1.charAt(i);
+                char c2 = word2.charAt(j);
+
+                // If the last character is the same, we do nothing compare to previous string
+                if (c1 == c2) {
+                    dp[i + 1][j + 1] = dp[i][j];
+                } else {
+                    // we chose the minimum operation of three operation: insert, delete, replace
+                    dp[i + 1][j + 1] = Math.min(dp[i][j], Math.min(dp[i+1][j], dp[i][j+1])) + 1;
+                }
+            }
+        }
+
+        return dp[m][n];
+    }
+}