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 ofeverynode never differ by more than 1.

Solution 1: 用-1表示unbalanced，> = 0表示高度

    public boolean isBalanced(TreeNode root) {
        return helper(root) != -1;
    }
    private int helper(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int left = helper(root.left);
        if (left == -1) {
            return -1;
        }
        int right = helper(root.right);
        if (right == -1) {
            return -1;
        }
        if (Math.abs(left - right) > 1) {
            return -1;
        }
        return Math.max(left, right) + 1;
    }

Solution 2: 对每个node求左右子树的高度，比较

  public boolean isBalanced(TreeNode root) {
    // Write your solution here.
    if (root == null) {
      return true;
    }
    if (Math.abs(getHeight(root.left) - getHeight(root.right)) > 1) {
      return false;
    }
    return isBalanced(root.left) && isBalanced(root.right);
  }
  private int getHeight(TreeNode root) {
    if (root == null) {
      return 0;
    }
    return Math.max(getHeight(root.left), getHeight(root.right)) + 1;
  }