二叉树最大深度

后序递归

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

先序递归

    private int result = 0;
public int maxDepth(TreeNode root) {
    if(root == null) {
        return result;
    }
    depth(root, 1);
    return result;
}

private void depth(TreeNode node, int depth) {
    if(node.left == null && node.right == null) {
        result = Math.max(result, depth);
        return;
    }
    if(node.left != null) {
        depth(node.left, depth + 1);
    }
    if(node.right != null) {
        depth(node.right, depth + 1);
    }
} 

迭代法

    public int maxDepth(TreeNode root) {
    Queue<TreeNode> queue = new LinkedList();
    int result = 0;
if(root == null) {
    return 0;
}
queue.offer(root);
while(!queue.isEmpty()) {
    int size = queue.size();
    result++;
    while(size-- > 0) {
        TreeNode tmp = queue.poll();
        if(tmp.left != null) {
            queue.offer(tmp.left);
        }
         if(tmp.right != null) {
            queue.offer(tmp.right);
        }
    }
}
return result;
}

最小深度

后序

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

先序

    private int result ;
public int minDepth(TreeNode root) {
     result = Integer.MAX_VALUE;
    if(root == null) {
        return 0;
    }
    depth(root, 1);
    return result;
}

private void depth(TreeNode node, int depth) {
    if(node.left == null && node.right == null) {
        result = Math.min(result, depth);
        return;
    }
    if(node.left != null) {
        depth(node.left, depth + 1);
    }
    if(node.right != null) {
        depth(node.right, depth + 1);
    }
} 

二叉树节点个数

    public int countNodes(TreeNode root) {
    if (root == null) return 0;
    TreeNode left = root.left;
    TreeNode right = root.right;
    int leftDepth = 0, rightDepth = 0; // 这里初始为0是有目的的，为了下面求指数方便
    while (left != null) {  // 求左子树深度
        left = left.left;
        leftDepth++;
    }
    while (right != null) { // 求右子树深度
        right = right.right;
        rightDepth++;
    }
    if (leftDepth == rightDepth) {
        return (2 << leftDepth) - 1; // 注意(2<<1) 相当于2^2，所以leftDepth初始为0
    }
    return countNodes(root.left) + countNodes(root.right) + 1;
}