平衡二叉树两种解法

自顶向下O(Nlog2​N)

class Solution {
    public boolean isBalanced(TreeNode root) {
        if (root == null) return true;
        int l = getHeight(root.left);
        int r = getHeight(root.right);
        if (Math.abs(l - r) > 1) return false;
        return isBalanced(root.left) && isBalanced(root.right);
    }

    public int getHeight(TreeNode root) {
        if (root == null) return 0;
        return Math.max(getHeight(root.left), getHeight(root.right)) + 1;
    }
}

自底向上O(N)

• 逐层返回高度和-1表示 不成立

class Solution {
    public boolean isBalanced(TreeNode root) {
        return recur(root) != -1;
    }

    public int recur(TreeNode root) {
        if (root == null) return 0;
        int left = recur(root.left);
        if (left == -1) return -1;
        int right = recur(root.right);
        if (right == -1) return -1;
        return Math.abs(left - right) > 1 ? -1 : Math.max(left, right) + 1;
    }
}