Merge pull request #331 from soapyigu/DP

Add a solution to the Longest Common Subsequence

Author: soapyigu

DP/LongestCommonSubsequence.swift

@@ -0,0 +1,26 @@
+/**
+ * Question Link: https://leetcode.com/problems/longest-common-subsequence/
+ * Primary idea: Dynamic Programming,  dp[i][j] = dp[i - 1][j - 1] + 1 or max(dp[i - 1][j], dp[i][j - 1])
+ *
+ * Time Complexity: O(mn), Space Complexity: O(1)
+ */
+
+class LongestCommonSubsequence {
+    func longestCommonSubsequence(_ text1: String, _ text2: String) -> Int {
+        let text1Chars = Array(text1), text2Chars = Array(text2)
+        let m = text1.count, n = text2.count
+        var dp = Array(repeating: Array(repeating: 0, count: n + 1), count: m + 1)
+        
+        for i in 1...m {
+            for j in 1...n {
+                if text1Chars[i - 1] == text2Chars[j - 1] {
+                    dp[i][j] = dp[i - 1][j - 1] + 1
+                } else {
+                    dp[i][j] = max(dp[i - 1][j], dp[i][j - 1])
+                }  
+            }
+        }
+        
+        return dp[m][n]   
+    }
+}