题解 | #判断是不是平衡二叉树#

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 {
    private Map<TreeNode, Integer> dh = new HashMap<>();
    /**
     * 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 pRoot TreeNode类 
     * @return bool布尔型
     */
    public boolean IsBalanced_Solution (TreeNode pRoot) {
        // write code here
        depth(pRoot);
        return isBalancedTree(pRoot);
    }
	// 重点是要构造深度映射关系
    private Integer depth(TreeNode root) {
        if (root == null) {
            return 0;
        }
        if (dh.containsKey(root)) return dh.get(root);
        int d = Math.max(depth(root.left), depth(root.right)) + 1;
        dh.put(root, d);
        return d;
    }

    private boolean isBalancedTree(TreeNode root) {
        if (root == null) {
            return true;
        }

        if (!isBalancedTree(root.left)) {
            return false;
        }


        if (!isBalancedTree(root.right)) {
            return false;
        }
        return Math.abs(depth(root.left) - depth(root.right)) <= 1;

    }
}