剑指Offer 54 - 二叉搜索树的第K大节点

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @param {number} k
 * @return {number}
 */
var kthLargest = function(root, k) {

    let res;
    Inorder(root);
    return res;
    
    function Inorder(root){
        if(!root)   return;
        
        //中序遍历右节点，若遍历后k为0，说明已经找到第k大的数，提前返回。
        Inorder(root.right);
        if(k === 0) return;
       
        //处理当前节点，若k-1=0，说明当前节点为第K大的节点，存下结果并返回。
       if(--k === 0){
           res = root.val;
           return;
       }
        
        //中序遍历左节点
        Inorder(root.left);
        
    }
    
};

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @param {number} k
 * @return {number}
 */
var kthLargest = function(root, k) {
    const stack = [];
    let count = 0;
    while(root || stack.length){
        while(root){
            stack.push(root);
            root = root.right;
        }
        
        const cur = stack.pop();
        if(++count === k){
            return cur.val;
        }
        
        root = cur.left;
    }
};