KthLargestElementInABST

Author: Guanchen Zhao

src/leetcode/solution/tree/bst/KthLargestElementInABST.java

@@ -0,0 +1,119 @@
+package leetcode.solution.tree.bst;
+
+import leetcode.structure.TreeNode;
+
+import java.util.Stack;
+
+/**
+ * Kth Largest Element in a BST
+ */
+public class KthLargestElementInABST {
+
+    public static void main(String[] args) {
+        Integer[] inorder = {3, 1, 10, null, 2, 7, null};
+        TreeNode treeNode = TreeNode.constructTree(inorder);
+        int k = 4;
+        KthLargestElementRecursion recursion = new KthLargestElementRecursion();
+        int ans = recursion.kthLargest(treeNode, k);
+        System.out.println(ans);
+
+
+        KthLargestElementIteration iteration = new KthLargestElementIteration();
+        ans = iteration.kthLargest(treeNode, k);
+        System.out.println(ans);
+
+
+        inorder = new Integer[]{5, 3, 10, null, null, 7, 15, null, null, 14, 16};
+        treeNode = TreeNode.constructTree(inorder);
+        k = 6;
+        ans = recursion.kthLargest(treeNode, k);
+        System.out.println(ans);
+
+
+        ans = iteration.kthLargest(treeNode, k);
+        System.out.println(ans);
+    }
+
+
+}
+
+class KthLargestElementRecursion {
+    /**
+     * global variable, same as k
+     * 全局变量，记录要找的排名
+     */
+    private int kth;
+
+    /**
+     * the order of current node
+     * 当前节点的排名
+     */
+    private int current;
+
+    /**
+     * the value of final result
+     * 结果值
+     */
+    private int value;
+
+
+    public int kthLargest(TreeNode root, int k) {
+        kth = k;
+        current = 0;
+        helper(root);
+        return value;
+    }
+
+    /**
+     * Inorder Traversal
+     * 中序遍历
+     * <p>
+     * means sort from smallest to largest in the Binary Search Tree
+     * 中序遍历即为二叉搜索树的从小到大排序
+     *
+     * @param root
+     */
+    private void helper(TreeNode root) {
+        if (root == null) {
+            return;
+        }
+
+        helper(root.right);
+
+        current++;
+
+        if (current == kth) {
+            value = root.val;
+            return;
+        }
+
+        helper(root.left);
+    }
+
+
+}
+
+
+class KthLargestElementIteration {
+    public int kthLargest(TreeNode root, int k) {
+        Stack<TreeNode> stack = new Stack<>();
+        TreeNode p = root;
+        int count = 0;
+
+        while (!stack.isEmpty() || p != null) {
+            if (p != null) {
+                stack.push(p);  // Just like recursion
+                p = p.right;
+
+            } else {
+                TreeNode node = stack.pop();
+                if (++count == k) {
+                    return node.val;
+                }
+                p = node.left;
+            }
+        }
+
+        return Integer.MIN_VALUE;
+    }
+}