Leetcode 110 balanced binary tree

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1. Balanced binary tree means that the height difference between the left and right subtrees does not exceed 1
2. Building recursive functions recur
3. When node by null, It will terminate directly
4. Here are the detailed operations of recursive functions ： Use left To save the height of the left subtree or whether it is -1, Then judge whether the left subtree is -1, If so, go back to -1, Similarly, right subtree . Finally back to ： Judge the height difference between the left and right subtrees Less than 2, If less than 2, Returns the height of the current number , Instead, return to - 1

class Solution {
    
    public boolean isBalanced(TreeNode root) {
    
        // At this time, if the return value of the difference between the left and right depths of the root node is not -1, That's right 
        return recur(root) != -1;
    }
    public int recur(TreeNode node){
    
        if(node == null) return 0;
        int left = recur(node.left);
        if(left == -1) return -1;
        int right = recur(node.right);
        if(right == -1) return -1;
        // If the depth difference is less than 2  Represents a balanced binary tree , Then return to the current maximum depth , Otherwise return to -1
        return Math.abs(left - right) < 2 ? Math.max(left, right) + 1 : -1;
    }
}