题目：

题解：

func isLeafNode(node *TreeNode) bool {
    return node.Left == nil && node.Right == nil
}

func sumOfLeftLeaves(root *TreeNode) (ans int) {
    if root == nil {
        return
    }
    q := []*TreeNode{root}
    for len(q) > 0 {
        node := q[0]
        q = q[1:]
        if node.Left != nil {
            if isLeafNode(node.Left) {
                ans += node.Left.Val
            } else {
                q = append(q, node.Left)
            }
        }
        if node.Right != nil && !isLeafNode(node.Right) {
            q = append(q, node.Right)
        }
    }
    return
}