404.求二叉树左叶子节点之和 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.

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

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