404. Sum of Left Leaves

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

左树的值（9+15=24）

/**
 * 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;

    }
}