404. Sum of Left Leaves

Find the sum of all left leaves in a given binary tree.

Example:

    3
   / \
  9  20
    /  \
   15   7

There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.

class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        if(root == null) return 0;
        int res = 0;
        if(root.left != null && root.left.left == null && root.left.right == null) res += root.left.val;
        return res + sumOfLeftLeaves(root.left) + sumOfLeftLeaves(root.right);
    }
}