404. 左叶子之和

404. 左叶子之和

/**
 * 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 dfs(root)
}
func isLeaf(node *TreeNode) bool {
    return node.Left == nil && node.Right == nil
}
func dfs(root *TreeNode) (ans int) {
    if root.Left != nil {
        if isLeaf(root.Left) {
            ans += root.Left.Val
        } else {
            ans += dfs(root.Left)
        }
    }
    if root.Right != nil && !isLeaf(root.Right) {
        ans += dfs(root.Right)
    }
    return
}