K-th node wins the offer-34. Binary search tree (269)

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34. The k-th node in a binary search tree (269)

  • Topic Description: Given a binary search tree, please find the first k smaller nodes therein. For example, (5,3,7,2,4,6,8), the numerical values ​​according to the third junction point is 4 summary.

  • Ideas: for a binary search tree traversal sequence, when the count of the access node. If the k-th access node is returned.

  • Code

    package _34.二叉搜索树中第k的结点;
    
    import java.util.Stack;
    
    /**
     * 题目描述:给定一棵二叉搜索树,请找出其中的第k小的结点。
     * 例如, (5,3,7,2,4,6,8)    中,按结点数值大小顺序第三小结点的值为4。
     * @author Administrator
     *
     */
    public class KthNodeInBST {
    	/**
    	 * 中序遍历到第k个结点
    	 * @param pRoot
    	 * @param k
    	 * @return
    	 */
    	static TreeNode KthNode(TreeNode pRoot, int k) {
    		if(pRoot == null) return null;
    		
    		Stack<TreeNode> stack = new Stack<TreeNode>();
    		TreeNode cur = pRoot;
    		
    		int i = 0;
    		while(!stack.isEmpty() || cur != null){
    			while(cur != null){
    				stack.push(cur);
    				cur = cur.left;
    			}
    			if(cur == null){
    				cur = stack.pop();
    				//访问
    				i++;
    				if(i == k){
    					return cur;
    				}
    				cur = cur.right;
    			}
    		}
    		
    		return null;
    	}
    	
    	public static void main(String[] args) {
    		/**
    		 *          5
    			      /   \
    			     3     7
    			    / \   / \
    			   2  4  6   8 
    		 */
    		TreeNode root = new TreeNode(5);
            root.left = new TreeNode(3);
            root.left.left = new TreeNode(2);
            root.left.right = new TreeNode(4);
            root.right = new TreeNode(7);
            root.right.left = new TreeNode(6);
            root.right.right = new TreeNode(8);
            System.out.println(KthNode(root,3).val);//4
            System.out.println(KthNode(root,6).val);//7
            System.out.println(KthNode(root,8));//null
    	}
    }
    
    class TreeNode {
    	int val = 0;
    	TreeNode left = null;
    	TreeNode right = null;
    
    	public TreeNode(int val) {
    		this.val = val;
    
    	}
    }
    

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