[LeetCode] 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.

Thought process:
Recursively add the sum of left leaves of root.left and root.right.

Solution:

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        if (root == null) {
            return 0;
        }
        
        int sum = 0;
        if (root.left != null) {
            if (root.left.left == null && root.left.right == null) {
                sum += root.left.val;
            } else {
                sum += sumOfLeftLeaves(root.left);
            }
        }
        if (root.right != null && (root.right.left != null || root.right.right != null)) {
            sum += sumOfLeftLeaves(root.right);
        }
        return sum;
    }
}

Time complexity: O(n).
Location: Philadelphia, PA, USA

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