解题思路：

后序遍历

左叶子节点：父节点的左节点不为空，该左节点的左右节点都为空

class Solution {public int sumOfLeftLeaves(TreeNode root) {return recur(root);}public int recur(TreeNode root) {if (root == null) return 0;if (root.left == null && root.right == null) return 0;int leftNum = recur(root.left);if (root.left != null && root.left.left == null && root.left.right == null)leftNum = root.left.val;int rightNum = recur(root.right);int res = leftNum + rightNum;return res;}
}