Java solution 155 && 173

Author: gavinfish

java/_155MinStack.java

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+import java.util.Stack;
+
+/**
+ * Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.
+ * <p>
+ * push(x) -- Push element x onto stack.
+ * pop() -- Removes the element on top of the stack.
+ * top() -- Get the top element.
+ * getMin() -- Retrieve the minimum element in the stack.
+ * <p>
+ * Example:
+ * MinStack minStack = new MinStack();
+ * minStack.push(-2);
+ * minStack.push(0);
+ * minStack.push(-3);
+ * minStack.getMin();   --> Returns -3.
+ * minStack.pop();
+ * minStack.top();      --> Returns 0.
+ * minStack.getMin();   --> Returns -2.
+ * <p>
+ * Created by drfish on 6/8/2017.
+ */
+public class _155MinStack {
+    public class MinStack {
+        Stack<Integer> stack;
+        Stack<Integer> minStack;
+
+        /**
+         * initialize your data structure here.
+         */
+        public MinStack() {
+            stack = new Stack<>();
+            minStack = new Stack<>();
+        }
+
+        public void push(int x) {
+            stack.push(x);
+            if (minStack.isEmpty() || minStack.peek() >= x) {
+                minStack.push(x);
+            }
+        }
+
+        public void pop() {
+            int x = stack.pop();
+            if (x == minStack.peek()) {
+                minStack.pop();
+            }
+        }
+
+        public int top() {
+            return stack.peek();
+        }
+
+        public int getMin() {
+            return minStack.peek();
+        }
+    }
+
+    public static void main(String[] args) {
+        MinStack minStack = new _155MinStack().new MinStack();
+        minStack.push(-2);
+        minStack.push(0);
+        minStack.push(-3);
+        assert -3 == minStack.getMin();
+        minStack.pop();
+        assert 0 == minStack.top();
+        assert -2 == minStack.getMin();
+    }
+}

java/_173BinarySearchTreeIterator.java

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+import java.util.Stack;
+
+/**
+ * Implement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.
+ * Calling next() will return the next smallest number in the BST.
+ * <p>
+ * Note: next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.
+ * <p>
+ * Credits:
+ * Special thanks to @ts for adding this problem and creating all test cases.
+ * <p>
+ * Created by drfish on 6/8/2017.
+ */
+public class _173BinarySearchTreeIterator {
+    public class BSTIterator {
+        private Stack<TreeNode> stack;
+        private TreeNode node;
+
+        public BSTIterator(TreeNode root) {
+            stack = new Stack<>();
+            node = root;
+        }
+
+        /**
+         * @return whether we have a next smallest number
+         */
+        public boolean hasNext() {
+            return node != null || !stack.isEmpty();
+        }
+
+        /**
+         * @return the next smallest number
+         */
+        public int next() {
+            while (node != null) {
+                stack.push(node);
+                node = node.left;
+            }
+            TreeNode temp = stack.pop();
+            node = temp.right;
+            return temp.val;
+        }
+    }
+}