/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func sumOfLeftLeaves(root *TreeNode) int {
	if root == nil {
		return 0
	}
	return sumRecurse(root, false)
}

func sumRecurse(root *TreeNode, isLeft bool) (sum int) {
	if root.Left == nil && root.Right == nil {
		if isLeft {
			return root.Val
		}
		return
	}
	if root.Left != nil {
		sum += sumRecurse(root.Left, true)
	}
	if root.Right != nil {
		sum += sumRecurse(root.Right, false)
	}
	return
}