题解 | #左叶子之和#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型
     */
    public int sumOfLeftLeaves(TreeNode root) {
        // write code here
        HashMap<TreeNode, TreeNode> fatherNodes = new HashMap<>();
        fatherNodes.put(root, root);
        return process(root, fatherNodes);
    }

    public int process(TreeNode node, HashMap<TreeNode, TreeNode> fatherNodes) {
        if (null == node) {
            return 0;
        }
        if (null == node.left && null == node.right) {
            TreeNode fatherNode = fatherNodes.get(node);
            if (fatherNode.left == node) {
                return node.val;
            } else {
                return 0;
            }
        }
        if (null != node.left) {
            fatherNodes.put(node.left, node);
        }
        if (null != node.right) {
            fatherNodes.put(node.right, node);
        }
        return process(node.left, fatherNodes) + process(node.right, fatherNodes);
    }
}