day17 110.平衡二叉树 257. 二叉树的所有路径 404.左叶子之和

110.平衡二叉树 257. 二叉树的所有路径 404.左叶子之和

110.平衡二叉树

class Solution {
    public boolean isBalanced(TreeNode root) {
        return getHeight(root) != -1;
    }
    public int getHeight(TreeNode root){
        if(root == null) return 0;
        int rightHeight = getHeight(root.right);
        if (rightHeight == -1) {
            return -1;
        }
        int leftHeight = getHeight(root.left);
        if (leftHeight == -1) {
            return -1;
        }
        if(Math.abs(rightHeight - leftHeight) > 1){ 
            return -1;
        }
        return Math.max(leftHeight, rightHeight) + 1;
    }
}

257. 二叉树的所有路径

class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> res = new ArrayList<>();// 存最终的结果
        if (root == null) {
            return res;
        }
        List<Integer> paths = new ArrayList<>();// 作为结果中的路径
        traversal(root, paths, res);
        return res;
    }
    private void traversal(TreeNode root, List<Integer> paths, List<String> res) {
        paths.add(root.val);
        if (root.left == null && root.right == null) {
            // 输出
            StringBuilder sb = new StringBuilder();// StringBuilder用来拼接字符串，速度更快
            for (int i = 0; i < paths.size() - 1; i++) {
                sb.append(paths.get(i)).append("->");
            }
            sb.append(paths.get(paths.size() - 1));// 记录最后一个节点
            res.add(sb.toString());// 收集一个路径
            return;
        }
        if (root.left != null) { // 左
            traversal(root.left, paths, res);
            paths.remove(paths.size() - 1);// 回溯
        }
        if (root.right != null) { // 左
            traversal(root.right, paths, res);
            paths.remove(paths.size() - 1);// 回溯
        }
    }
}

404.左叶子之和

class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        if(root == null) return 0;
        int leftValue = sumOfLeftLeaves(root.left);
        int rightValue = sumOfLeftLeaves(root.right);
        int midValue = 0;
        if(root.left != null && root.left.left == null && root.left.right == null)  midValue = root.left.val;
        
        return midValue + leftValue + rightValue;
    }
}