class Solution {
   /**
     * 优化迭代法，针对暴力迭代法的getHeight方法做优化，利用TreeNode.val来保存当前结点的高度，这样就不会有重复遍历
     * 获取高度算法时间复杂度可以降到O(1)，总的时间复杂度降为O(n)。
     * 时间复杂度：O(n)
     */
    public boolean isBalanced(TreeNode root) {
        if (root == null) {
            return true;
        }
        Stack<TreeNode> stack = new Stack<>();
        TreeNode pre = null;
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            TreeNode inNode = stack.peek();
            // 右结点为null或已经遍历过
            if (inNode.right == null || inNode.right == pre) {
                // 输出
                if (Math.abs(getHeight(inNode.left) - getHeight(inNode.right)) > 1) {
                    return false;
                }
                stack.pop();
                pre = inNode;
                root = null;// 当前结点下，没有要遍历的结点了
            } else {
                root = inNode.right;// 右结点还没遍历，遍历右结点
            }
        }
        return true;
    }

    /**
     * 求结点的高度
     */
    public int getHeight(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int leftHeight = root.left != null ? root.left.val : 0;
        int rightHeight = root.right != null ? root.right.val : 0;
        int height = Math.max(leftHeight, rightHeight) + 1;
        root.val = height;// 用TreeNode.val来保存当前结点的高度
        return height;
    }
}

class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> result = new ArrayList<>();
        if (root == null)
            return result;
        Stack<Object> stack = new Stack<>();
        stack.push(root);
        stack.push(root.val + "");
        while (!stack.isEmpty()) {
            String path = (String) stack.pop();
            TreeNode node = (TreeNode) stack.pop();
            if (node.left == null && node.right == null) {
                result.add(path);
            }
            if (node.right != null) {
                stack.push(node.right);
                stack.push(path + "->" + node.right.val);
            }
            if (node.left != null) {
                stack.push(node.left);
                stack.push(path + "->" + node.left.val);
            }
        }
        return result;
    }
}

class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        if (root == null) return 0;
        Stack<TreeNode> stack = new Stack<> ();
        stack.add(root);
        int result = 0;
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            if (node.left != null && node.left.left == null && node.left.right == null) {
                result += node.left.val;
            }
            if (node.right != null) stack.add(node.right);
            if (node.left != null) stack.add(node.left);
        }
        return result;
    }
}