404.java

/**
 * 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 {
    private int ans = 0;

    public int sumOfLeftLeaves(TreeNode root) {
        if (root == null)
            return 0;
        dfs(root);
        return ans;
    }

    private void dfs(TreeNode node) {
        if(node==null){
            return ;
        }
        if (node.left != null) {
            if (node.left.left == node.left.right) {
                ans += node.left.val;
            }
        }
        dfs(node.left);
        dfs(node.right);
    }
}