404. 左叶子之和

404. 左叶子之和

原题

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    int sum = 0;
    public int sumOfLeftLeaves(TreeNode root) {
        //第二个参数代表当前结点是左节点（1），还是右节点（0），一开始传0，让头结点不参与运算
        dfs(root,0);
        return sum;
    }

    public void dfs(TreeNode node,int i){
        if(node==null){
            return;
        }
        //为叶子结点
        if(node.left==null&&node.right==null){
            //为左叶子节点
            if(i==1){
                sum += node.val;
            }
        }
        //左子树递归
        dfs(node.left,1);
        //右子树递归
        dfs(node.right,0);
    }
}