cs: add «139. Word Break»

URL:	https://leetcode.com/problems/word-break/
Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/word-break.cs

@@ -0,0 +1,18 @@
+public class Solution {
+    public bool WordBreak(string s, IList<string> wordDict) {
+        var words = new HashSet<string>(wordDict);
+        var dp = new bool[s.Length + 1];
+
+        dp[0] = true;
+        for (var i = 1; i <= s.Length; ++i) {
+            for (var j = 0; j < i; ++j) {
+                if (dp[j] && words.Contains(s.Substring(j, i - j))) {
+                    dp[i] = true;
+                    break;
+                }
+            }
+        }
+
+        return dp[s.Length];
+    }
+}