404. Sum of Left Leaves

Find the sum of all left leaves in a given binary tree.

Example:

    3
   / \
  9  20
    /  \
   15   7

There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.
题目含义：获取所有左叶子节点的总和
方法一：

    public int sumOfLeftLeaves(TreeNode root) {
        if (root == null) return 0;
        int ans = 0;
        if (root.left != null) {
            if (root.left.left == null && root.left.right == null) ans += root.left.val;
            else ans += sumOfLeftLeaves(root.left);
        }
        ans += sumOfLeftLeaves(root.right);
        return ans;
    }

方法二：

    public int sumOfLeftLeaves(TreeNode root) {
        if (root == null) return 0;
        int ans = 0;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        while (!stack.empty()) {
            TreeNode node = stack.pop();
            if (node.left != null) {
                if (node.left.left == null && node.left.right == null) ans += node.left.val;
                else stack.push(node.left);
            }
            if (node.right != null) {
                if (node.right.left == null && node.right.right == null) continue;
                else stack.push(node.right);
            }
        }
        return ans;
    }