二叉树

继续把昨天剩的两道打完。

104. 二叉树的最大深度

层次遍历迭代

class Solution {
    public int maxDepth(TreeNode root) {
        int depth = 0;
        Deque<TreeNode> queue = new LinkedList<>();
        if (root != null) queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode cur = queue.poll();
                if (cur.left != null) queue.offer(cur.left);
                if (cur.right != null) queue.offer(cur.right);
            }
            depth++;
        }
        return depth;
    }
}

后序遍历递归

class Solution {
    public int maxDepth(TreeNode root) {
        return getDepth(root, 0);
    }
    private int getDepth(TreeNode root, int depth) {
        if (root == null) return 0;
        //左右中
        return Math.max(getDepth(root.left, depth), getDepth(root.right, depth)) + 1;
    }
}

111. 二叉树的最小深度

class Solution {
    public int minDepth(TreeNode root) {
        int res = 0;
        Deque<TreeNode> queue = new LinkedList<>();
        if (root != null) {
            queue.offer(root);
            res++; //包括自身
        }
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode cur = queue.poll();
                //存在无子结点的结点，找到最小深度
                if (cur.left == null && cur.right == null) return res;
                if (cur.left != null) queue.offer(cur.left);
                if (cur.right != null) queue.offer(cur.right);
            }
            res++; //该层不存在无子结点的结点。
        }
        //空树返回
        return res;
    }
}

同样可以使用递归

class Solution {
    public int minDepth(TreeNode root) {
        return getDepth(root, 0);
    }
    private int getDepth(TreeNode root, int depth) {
        if (root == null) return 0;
        //左右中 null + right + mid  left + null + mid left + right + mid
        if (root.left == null && root.right != null) return getDepth(root.right, depth) + 1;
        if (root.left != null && root.right == null) return getDepth(root.left, depth) + 1;
        return Math.min(getDepth(root.left, depth), getDepth(root.right, depth)) + 1;
    }
}

参考：代码随想录：programmercarl.com