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方法二 自底向上

class Solution {
       public boolean isBalanced(TreeNode root) {
           return height(root) >= 0;    }    public int height(TreeNode root) {
           if (root == null) {
               return 0;        }        int leftHeight = height(root.left);        int rightHeight = height(root.right);        if (leftHeight == -1 || rightHeight == -1 || Math.abs(leftHeight - rightHeight) > 1) {
               return -1;        } else {
               return Math.max(leftHeight, rightHeight) + 1;        }    }}

方法一 自顶向下

/** * 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 {
       public boolean isBalanced(TreeNode root) {
           if(root == null) {
               return true;        }		//先判断当前节点  然后判断左右节点        return Math.abs(height(root.left) - height(root.right)) <= 1 && isBalanced(root.left) && isBalanced(root.right);    }    public int help(TreeNode root) {
           if(root == null) {
               return 0;        }        int left = height(root.left);        int right = height(root.right);        return Math.max(left, right) + 1;    }}

这个也是自底向上

/** * 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 {
       public boolean isBalanced(TreeNode root) {
           if(root == null) {
               return true;        }		//判断当前节点时，先判断左右节点        return isBalanced(root.left) && isBalanced(root.right) && Math.abs(help(root.left) - help(root.right)) <= 1;    }    public int help(TreeNode root) {
           if(root == null) {
               return 0;        }        int left = help(root.left);        int right = help(root.right);        return Math.max(left, right) + 1;    }}

用层序遍历的方法求二叉树的高度

/** * 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 {
       public boolean isBalanced(TreeNode root) {
           if(root == null) {
               return true;        }        return isBalanced(root.left) && isBalanced(root.right) && Math.abs(help(root.left) - help(root.right)) <= 1;    }    public int help(TreeNode root) {
           if(root == null) {
               return 0;        }        Queue
   
     queue = new LinkedList<>();        queue.offer(root);        int res = 0;        while(!queue.isEmpty()) {
               int size = queue.size();            while(size > 0) {
                   TreeNode cur = queue.poll();                size--;                if(cur.left != null) {
                       queue.offer(cur.left);                }                if(cur.right != null) {
                       queue.offer(cur.right);                }            }            //执行到这里一行就执行完了            res++;                    }        return res;    }}
   

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