Sum of Left Leaves
Given the root of a binary tree, return the sum of all left leaves.
A leaf is a node with no children. A left leaf is a leaf that is the left child of another node.
Example 1:
Input: root = [3,9,20,null,null,15,7]
Output: 24
Explanation: There are two left leaves in the binary tree, with values 9 and 15 respectively.
Example 2:
Input: root = [1]
Output: 0
Constraints:
The number of nodes in the tree is in the range [1, 1000].
-1000 <= Node.val <= 1000
思路一：递归，递归的结束条件是 left Leaves 的判断条件，即没有左右两个子节点，刚开始做的时候没有仔细看题，右边的子节点也要判空
public int sumOfLeftLeaves(TreeNode root) {
return sumOfLeftLeaves2(root.left, true) + sumOfLeftLeaves2(root.right, false);
}
public int sumOfLeftLeaves2(TreeNode root, boolean isLeft) {
if (root == null) return 0;
if (root.left == null && root.right == null && isLeft) {
return root.val;
}
return sumOfLeftLeaves2(root.left, true) + sumOfLeftLeaves2(root.right, false);
}