Java solution 331 && 341

Author: gavinfish

java/_331VerifyPreorderSerializationOfABinaryTree.java

@@ -0,0 +1,58 @@
+import java.util.Stack;
+
+/**
+ * One way to serialize a binary tree is to use pre-order traversal. When we encounter a non-null node, we record the node's value. If it is a null node, we record using a sentinel value such as #.
+ * <p>
+ *      _9_
+ *     /   \
+ *    3     2
+ *   / \   / \
+ *  4   1  #  6
+ * / \ / \   / \
+ * # # # #   # #
+ * For example, the above binary tree can be serialized to the string "9,3,4,#,#,1,#,#,2,#,6,#,#", where # represents a null node.
+ * Given a string of comma separated values, verify whether it is a correct preorder traversal serialization of a binary tree. Find an algorithm without reconstructing the tree.
+ * Each comma separated value in the string must be either an integer or a character '#' representing null pointer.
+ * You may assume that the input format is always valid, for example it could never contain two consecutive commas such as "1,,3".
+ * <p>
+ * Example 1:
+ * "9,3,4,#,#,1,#,#,2,#,6,#,#"
+ * Return true
+ * <p>
+ * Example 2:
+ * "1,#"
+ * Return false
+ * <p>
+ * Example 3:
+ * "9,#,#,1"
+ * Return false
+ * <p>
+ * Credits:
+ * Special thanks to @dietpepsi for adding this problem and creating all test cases.
+ * <p>
+ * Created by drfish on 6/9/2017.
+ */
+public class _331VerifyPreorderSerializationOfABinaryTree {
+    public boolean isValidSerialization(String preorder) {
+        Stack<String> stack = new Stack<>();
+        String[] splits = preorder.split(",");
+        for (String s : splits) {
+            while (s.equals("#") && !stack.isEmpty() && stack.peek().equals("#")) {
+                stack.pop();
+                if (stack.isEmpty()) {
+                    return false;
+                }
+                stack.pop();
+            }
+            stack.push(s);
+        }
+        return stack.size() == 1 && stack.peek().equals("#");
+    }
+
+    public static void main(String[] args) {
+        _331VerifyPreorderSerializationOfABinaryTree solution = new _331VerifyPreorderSerializationOfABinaryTree();
+        assert solution.isValidSerialization("9,3,4,#,#,1,#,#,2,#,6,#,#");
+        assert !solution.isValidSerialization("1,#");
+        assert !solution.isValidSerialization("9,#,#,1");
+    }
+}

java/_341FlattenNestedListIterator.java

@@ -0,0 +1,67 @@
+import java.util.Iterator;
+import java.util.List;
+import java.util.Stack;
+
+/**
+ * Given a nested list of integers, implement an iterator to flatten it.
+ * Each element is either an integer, or a list -- whose elements may also be integers or other lists.
+ * <p>
+ * Example 1:
+ * Given the list [[1,1],2,[1,1]],
+ * By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].
+ * <p>
+ * Example 2:
+ * Given the list [1,[4,[6]]],
+ * By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].
+ * <p>
+ * Created by drfish on 6/9/2017.
+ */
+public class _341FlattenNestedListIterator {
+    /**
+     * // This is the interface that allows for creating nested lists.
+     * // You should not implement it, or speculate about its implementation
+     **/
+    public interface NestedInteger {
+
+        // @return true if this NestedInteger holds a single integer, rather than a nested list.
+        public boolean isInteger();
+
+        // @return the single integer that this NestedInteger holds, if it holds a single integer
+        // Return null if this NestedInteger holds a nested list
+        public Integer getInteger();
+
+        // @return the nested list that this NestedInteger holds, if it holds a nested list
+        // Return null if this NestedInteger holds a single integer
+        public List<NestedInteger> getList();
+    }
+
+    public class NestedIterator implements Iterator<Integer> {
+        Stack<NestedInteger> stack = new Stack<>();
+
+        public NestedIterator(List<NestedInteger> nestedList) {
+            for (int i = nestedList.size() - 1; i >= 0; i--) {
+                stack.push(nestedList.get(i));
+            }
+        }
+
+        @Override
+        public Integer next() {
+            return stack.pop().getInteger();
+        }
+
+        @Override
+        public boolean hasNext() {
+            while (!stack.isEmpty()) {
+                NestedInteger curr = stack.peek();
+                if (curr.isInteger()) {
+                    return true;
+                }
+                stack.pop();
+                for (int i = curr.getList().size() - 1; i >= 0; i--) {
+                    stack.push(curr.getList().get(i));
+                }
+            }
+            return false;
+        }
+    }
+}