[Modification of in-order traversal] 230. The Kth smallest element in the binary search tree

230. Kth smallest element in binary search tree

Problem-solving ideas

• Transforming the in-order traversal algorithm
• For in-order traversal, each time a node is traversed, rank++ represents the order of the currently traversed elements and records the value of the currently traversed node.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    
    
    // 记录结果

    int res = 0;
    int rank = 0;
    public int kthSmallest(TreeNode root, int k) {
    
    
        traverse(root,k);

        return res;
    }

    public void traverse(TreeNode root,int k){
    
    
        if(root == null){
    
    
            return;
        }
        traverse(root.left,k);
        // 改造中序遍历算法
        rank++;
        if(k == rank){
    
    
            // 找到第k小的元素
            res = root.val;
            return;
        }
        traverse(root.right,k);
    }
}