Solution.java

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) {
        val = x;
    }
}

// Recursive + getHeight
class Solution {
    public boolean isBalanced(TreeNode root) {
        if (root == null) {
            return true;
        }
        return isBalanced(root.left) && isBalanced(root.right) && Math.abs(getHeight(root.left) - getHeight(root.right)) < 2;

    }

    public int getHeight(TreeNode root){
        if (root == null){
            return 0;
        }
        return 1 + Math.max(getHeight(root.left), getHeight(root.right));
    }
}

// Recursive
class Solution {
    public boolean isBalanced(TreeNode root) {
        return dfsHeight(root) != -1;
    }

    public int dfsHeight(TreeNode root){
        if (root == null){
            return 0;
        }
        int leftHeight = dfsHeight(root.left);
        if (leftHeight == -1) {
            return -1;
        }
        int rightHeight = dfsHeight(root.right);
        if (rightHeight == -1){
            return -1;
        }
        if (Math.abs(leftHeight - rightHeight) > 1) {
            return -1;
        }
        return 1 + Math.max(leftHeight, rightHeight);
    }
}