Sum of Left Leaves

https://leetcode.com/problems/sum-of-left-leaves/description/

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.

Thoughts

检查当前结点的左子树是不是叶子.

Code

/**
 * 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) {
        int sum = 0;
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            if (node != null) {
                if (node.left != null && node.left.left == null && node.left.right == null) {
                    sum += node.left.val;
                } 
                stack.push(node.left);
                stack.push(node.right);
            }
        }

        return sum;
    }
}

Analysis

时间复杂度O(N).