Balanced Binary Tree

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public boolean isBalanced(TreeNode root) {        if(root==null)            return true;        int l = height(root.left);        int r = height(root.right);        if(Math.abs(l-r)>1)            return false;        return isBalanced(root.left) && isBalanced(root.right);    }        public int height(TreeNode root){        if(root == null)            return 0;        if(root.left == null && root.right == null)            return 1;        int l = height(root.left);        int r = height(root.right);        return Math.max(l,r) + 1;    }}

 

 

So I thought of bottom-up, that is, post-sequential traversal.

 

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    boolean check = true;    public boolean isBalanced(TreeNode root) {        if(root==null)            return true;        int l = height(root.left);        int r = height(root.right);        if(Math.abs(l-r)>1)            return false;        return check;    }        public int height(TreeNode root){        if(root == null)            return 0;        if(root.left == null && root.right == null)            return 1;        int l = height(root.left);        int r = height(root.right);        if(Math.abs(l-r)>1)          check = false;        return Math.max(l,r) + 1;    }}

However, if height () return height does not need to be configured, return false. There is a very clever method below.

 

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    boolean check = true;    public boolean isBalanced(TreeNode root) {        if(root==null)            return true;        if(getHeight(root)== -1)            return false;        return true;    }        public int getHeight(TreeNode root){        if(root == null)            return 0;               int l = getHeight(root.left);        int r = getHeight(root.right);        if(l == -1 || r == -1)            return -1;        if(Math.abs(l-r)>1)          return -1;        return Math.max(l,r) + 1;    }}

 

Balanced Binary Tree