题目：

题解：

class Solution {public int sumOfLeftLeaves(TreeNode root) {if (root == null) {return 0;}Queue<TreeNode> queue = new LinkedList<TreeNode>();queue.offer(root);int ans = 0;while (!queue.isEmpty()) {TreeNode node = queue.poll();if (node.left != null) {if (isLeafNode(node.left)) {ans += node.left.val;} else {queue.offer(node.left);}}if (node.right != null) {if (!isLeafNode(node.right)) {queue.offer(node.right);}}}return ans;}public boolean isLeafNode(TreeNode node) {return node.left == null && node.right == null;}
}