Given a binary tree, determine if it is height-balanced.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of everynode never differ by more than 1.

public boolean isBalanced(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(isBalancedHelper(root)==-1){
            return false;
        }else{
            return true;
        }
    }

    //return -1 if the two subtrees of this node is unbalanced
    //return the height of this node otherwise
    public int isBalancedHelper(TreeNode node){
        if(node==null){
            return 0;
        }else{
            int leftHeight = isBalancedHelper(node.left);
            int rightHeight = isBalancedHelper(node.right);
            if(leftHeight==-1 || rightHeight==-1){
                return -1;
            }
            if(Math.abs(leftHeight-rightHeight)<=1){
                return Math.max(leftHeight,rightHeight)+1;
            }else{
                return -1;
            }
        }
    }

Running time is O(n) since all the nodes are visited only once.

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