Add more solutions

Author: rayworks

src/main/java/org/sean/stack/ParenthesesValidator.java

@@ -0,0 +1,44 @@
+package org.sean.stack;
+
+import java.util.Stack;
+
+// 20. Valid Parentheses
+public class ParenthesesValidator {
+    public boolean isValid(String s) {
+        if (s == null) {
+            return false;
+        }
+
+        Stack<Character> stack = new Stack<>();
+        for (int i = 0; i < s.length(); i++) {
+            char ch = s.charAt(i);
+            if (ch == '(' || ch == '{' || ch == '[') {
+                stack.push(ch);
+            }
+
+            if (ch == ')' || ch == '}' || ch == ']') {
+                if (stack.isEmpty()) {
+                    return false;
+                } else {
+                    Character outCh = stack.pop();
+                    switch (ch) {
+                        case ')':
+                            if (outCh != '(')
+                                return false;
+                            break;
+                        case '}':
+                            if (outCh != '{')
+                                return false;
+                            break;
+                        case ']':
+                            if (outCh != '[')
+                                return false;
+                            break;
+                    }
+                }
+            }
+        }
+
+        return stack.isEmpty();
+    }
+}

src/main/java/org/sean/tree/TreeMerger.java

@@ -0,0 +1,33 @@
+package org.sean.tree;
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ * int val;
+ * TreeNode left;
+ * TreeNode right;
+ * TreeNode(int x) { val = x; }
+ * }
+ */
+// 617. Merge Two Binary Trees
+class TreeMerger {
+
+    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
+        if (t1 == null && t2 == null) {
+            return null;
+        }
+
+        if (t1 == null)
+            return t2;
+
+        if (t2 == null)
+            return t1;
+
+        t1.val += t2.val;
+
+        t1.left = mergeTrees(t1.left, t2.left);
+        t1.right = mergeTrees(t1.right, t2.right);
+
+        return t1;
+    }
+}

src/test/java/org/sean/stack/ParenthesesValidatorTest.java

@@ -0,0 +1,22 @@
+package org.sean.stack;
+
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+
+public class ParenthesesValidatorTest {
+
+    private ParenthesesValidator validator;
+
+    @Before
+    public void setUp() throws Exception {
+        validator = new ParenthesesValidator();
+    }
+
+    @Test
+    public void isValid() {
+        Assert.assertTrue(validator.isValid(""));
+        Assert.assertFalse(validator.isValid("([)]"));
+        Assert.assertTrue(validator.isValid("{[]}"));
+    }
+}