404. 左叶子之和

dfs

/**
 * 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) {
        return dfs(root, false);
    }

    public int dfs(TreeNode node, boolean flag) {
        if(node == null){
            return 0;
        }
        if(node != null && node.left==null && node.right ==null){
           if(flag){
               return node.val;
           } else {
               return 0;
           }
        }
        int left = 0, right = 0;
        left += dfs(node.left, true);
        right += dfs(node.right, false);
        return left + right;
    }
}