/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @param {number} key
 * @return {TreeNode}
 
 
*/
var deleteNode = function(root, key) {
    if(!root){
        return null;
    }
    if(root.val == key){
        //find the node
        if(!root.left){
            return root.right;
        }else if(!root.right){
            return root.left;
        }else if (root.left&&root.right){
            
 
//find most left node;
            let prev = root;
            let next = root.right;
            while(next.left){
                prev = next;
                next = next.left;
            }
            //此時next就是當前右子樹的最左節點 
            root.val = next.val;
            if(prev==root){
                prev.right 
 
= next.right;
            }else{
                prev.left = next.right;
            }
        }
    }else{
        if(key<root.val){
            root.left = deleteNode(root.left,key)
        }else{
            root.right = deleteNode(root.right,key)
        }
    }
    return root;
};