/*
获取第K层节点的个数
*/
int getKLevelNodeCount(TreeNode root, int k) {
if(root == null){
return 0;
}
if (k == 1){
return 1;
}
int leftCount = getKLevelNodeCount(root.left,k-1);
int rightCount = getKLevelNodeCount(root.right,k-1);
return leftCount + rightCount;
}