BalancedBinaryTree.java

public boolean isBalanced(TreeNode root) {
		return maxHeight(root) != -1;
	}
	
	// if -1, means it is not a balanced tree, since it will also return the normal height(int), so boolean is not an option. 
	// Kinda of a hack for the return type.
	// @return -1 means it's already not a balanced tree, else t;he tree height
	public int maxHeight(TreeNode root) {
		if (root == null) {
			return 0;
		}
		
		int l = maxHeight(root.left);
		int r = maxHeight(root.right);

		// a classic usage of post order traversalß
		if (l == -1 || r == -1 || Math.abs(l - r) > 1) {
			return -1;
		}
		return Math.max(l, r) + 1;
	}