LeetCode 110. Balanced Binary Tree

最新推荐文章于 2023-11-11 23:37:07 发布

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本文介绍了两种检查二叉树是否平衡的递归算法：顶向下递归和底向上递归。通过计算每个子树的高度并比较左右子树高度差是否超过1来判断树是否平衡。顶向下递归先计算高度再判断平衡性，而底向上递归在计算高度的同时进行判断。

Top-down recursion

/**
 * 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 {
    public boolean isBalanced(TreeNode root) {
        if(root == null){
            return true;
        }
        int left = getHeight(root.left);
        int right = getHeight(root.right);
        if(Math.abs(left - right) > 1){
            return false;
        }
        return isBalanced(root.left) && isBalanced(root.right);
    }
    
    private int getHeight(TreeNode root){
        if(root == null){
            return 0;
        }
        int leftHeight = getHeight(root.left);
        int rightHeight = getHeight(root.right);
        int height = Math.max(leftHeight, rightHeight) + 1;
        return height;
    }
}

Bottom-up recursion

/**
 * 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 {
    public boolean isBalanced(TreeNode root) {
        return checkDepth(root) != -1;
    }
    
    private int checkDepth(TreeNode root){
        if(root == null){
            return 0;
        }
        int left = checkDepth(root.left);
        if(left == -1){
            return -1;
        }
        int right = checkDepth(root.right);
        if(right == -1){
            return -1;
        }
        if(Math.abs(left - right) < 2){
            return Math.max(left, right) + 1;
        }else{
            return -1;
        }
    }
}