原题:#404_左叶子之和
- 递归:
- 返回值:返回左子树的左叶子+右子树的左叶子之和
- 递归操作:遍历左子树和右子树,找到叶子节点并相加
- 结束条件:根节点为空时结束递归
public int sumOfLeftLeaves(TreeNode root) {
if (root == null) return 0;
if (isLeaf(root.left)) return root.left.val + sumOfLeftLeaves(root.right);
return sumOfLeftLeaves(root.left) + sumOfLeftLeaves(root.right);
}
private boolean isLeaf(TreeNode node){
if (node == null) return false;
return node.left == null && node.right == null;
}