題目

給定一顆二叉搜尋樹，請找出其中的第k大的結點。
例如，
5
/ \
3 7
/\ /\
2 4 6 8
中，按結點數值大小順序第三個結點的值為4。

思路

二叉搜尋樹：左子節點 <= 根節點 <= 右子節點
中序遍歷：左 -> 根 -> 右

程式碼

/**
 * 給定一顆二叉搜尋樹，請找出其中的第k大的結點。
 * 例如， 5 / \ 3 7 /\ /\ 2 4 6 8 中，按結點數值大小順序第三個結點的值為4。
 * 
 * @author peige
 */
public class _54_KthNodeInBST {

    public
 
 static class TreeNode {
        public int val = 0;
        public TreeNode left = null;
        public TreeNode right = null;
        public TreeNode(int val) {
            this.val = val;
        }
    }

    private int kth = 0;

    public TreeNode KthNode(TreeNode pRoot, int k) {
        if(pRoot == null
 
 || k <= 0)
            return null;
        TreeNode result = KthNode(pRoot.left, k);
        if(kth == k)
            return result;
        if(++kth == k)
            return pRoot;
        result = KthNode(pRoot.right, k);
        return result;
    }
}

測試

public class _54_Test {

    public
 
 static void main(String[] args) {
        test1();
        test2();
        test3();
    }

    /**
     *        8
     *      /   \
     *     6    12
     *    / \   / \
     *   3  7  10 14
     */
    private static void test1() {
        TreeNode node1 = new TreeNode(8);
        TreeNode node2 = new TreeNode(6);
        TreeNode node3 = new TreeNode(12);
        TreeNode node4 = new TreeNode(3);
        TreeNode node5 = new TreeNode(7);
        TreeNode node6 = new TreeNode(10);
        TreeNode node7 = new TreeNode(14);
        node1.left = node2;
        node1.right = node3;
        node2.left = node4;
        node2.right = node5;
        node3.left = node6;
        node3.right = node7;

        _54_KthNodeInBST knib = new _54_KthNodeInBST();
        TreeNode n = knib.KthNode(node1, 3);
        System.out.println(n == node5);

        _54_KthNodeInBST knib2 = new _54_KthNodeInBST();
        n = knib2.KthNode(node1, 4);
        System.out.println(n == node1);

        _54_KthNodeInBST knib3 = new _54_KthNodeInBST();
        n = knib3.KthNode(node1, 1);
        System.out.println(n == node4);

        _54_KthNodeInBST knib4 = new _54_KthNodeInBST();
        n = knib4.KthNode(node1, 7);
        System.out.println(n == node7);

        _54_KthNodeInBST knib5 = new _54_KthNodeInBST();
        n = knib5.KthNode(node1, 8);
        System.out.println(n);

        _54_KthNodeInBST knib6 = new _54_KthNodeInBST();
        n = knib6.KthNode(node1, 0);
        System.out.println(n);
    }

    private static void test2() {
        TreeNode node1 = new TreeNode(1);
        _54_KthNodeInBST knib = new _54_KthNodeInBST();
        TreeNode n = knib.KthNode(node1, 1);
        System.out.println(node1 == n);
    }

    private static void test3() {
        _54_KthNodeInBST knib = new _54_KthNodeInBST();
        TreeNode n = knib.KthNode(null, 1);
        System.out.println(n);
    }
}