题解 | #完全二叉树结点数#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

/**
 * NC84 完全二叉树结点数
 * @author d3y1
 */
public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param head TreeNode类 
     * @return int整型
     */
    public int nodeNum (TreeNode head) {
        // return solution1(head);
        // return solution2(head);
        return solution3(head);
    }

    /**
     * 前序遍历
     * @param head
     * @return
     */
    private int solution1(TreeNode head){
        return preOrder1(head);
    }

    private int preOrder1(TreeNode head){
        if(head == null){
            return 0;
        }

        return 1+preOrder1(head.left)+preOrder1(head.right);
    }

    /**
     * 前序遍历: 优化
     * @param head
     * @return
     */
    private int solution2(TreeNode head){
        return preOrder2(head);
    }

    private int preOrder2(TreeNode head){
        if(head == null){
            return 0;
        }

        int leftHeight=0;
        int rightHeight=0;
        // 当前树 最左节点高度
        for(TreeNode node=head; node!=null; node=node.left){
            leftHeight++;
        }
        // 当前树 最右节点高度
        for(TreeNode node=head; node!=null; node=node.right){
            rightHeight++;
        }
        // 当前树 满二叉树
        if(leftHeight == rightHeight){
            return (int)Math.pow(2.0, rightHeight)-1;
            // return (1<<rightHeight)-1;
        }

        return 1+preOrder2(head.left)+preOrder2(head.right);
    }

    /**
     * 前序遍历: 优化
     * @param head
     * @return
     */
    private int solution3(TreeNode head){
        return preOrder3(head);
    }

    private int preOrder3(TreeNode head){
        if(head == null){
            return 0;
        }

        int leftHeight=0;
        int rightHeight=0;
        // 左子树高度
        for(TreeNode node=head.left; node!=null; node=node.left){
            leftHeight++;
        }
        // 右子树高度
        for(TreeNode node=head.right; node!=null; node=node.left){
            rightHeight++;
        }
        // 左子树 满二叉树
        if(leftHeight == rightHeight){
            return 1+((1<<leftHeight)-1)+preOrder3(head.right);
        }
        // 右子树 满二叉树
        else{
            return 1+preOrder3(head.left)+((1<<rightHeight)-1);
        }
    }
}