/**

 * Definition for a binary tree node.

 * public class TreeNode {

 *     int val;

 *     TreeNode left;

 *     TreeNode right;

 *     TreeNode(int x) { val = x; }

 * }

 */

public class Solution {

    public int sumOfLeftLeaves(TreeNode root) {

        if(root==null){

            return 0;

        };

        int ans=0;

        Stack<TreeNode> stack=new Stack<TreeNode>();

        stack.push(root);

        while(!stack.empty()){

            TreeNode node=stack.pop();

            if(node.left!=null&&node.left.left==null&&node.left.right==null){

                    ans+=node.left.val;

                }

           if(node.right!=null){

                       stack.push(node.right);

            }

            if(node.left!=null){

                       stack.push(node.left);

            }

        }

         return ans;

    }

}