剑指Offer之Java算法习题精讲二叉树专题篇下

class Solution {
    public int maxDepth(Treenode root) {
        return  method(root);
    }
    int method(TreeNode root){
        if(root==null){
            return 0;
        }
        int l = method(root.left);
        int r = method(root.right);
        return Math.max(l, r) + 1;
    }
}

class Solution {
    int ans = 0;
    public int diameterOfBinaryTree(TreeNode root) {
        method(root);
        return ans;
    }
    public int method(TreeNode root){
        if(root==null){
            return 0;
        }
        int l = method(root.left);
        int r = method(root.right);
        ans = Math.max(ans,l+r);
        return Math.max(l,r)+1;
    }
}

class Solution {
    public int minDepth(TreeNode root) {
        if(root==null) return 0;
        if(root.left==null&&root.right==null) return 1;
        int min = Integer.MAX_VALUE;
        if(root.left!=null){
            min = Math.min(min,minDepth(root.left));
        }
        if(root.right!=null){
            min = Math.min(min,minDepth(root.right));
        }
        return min+1;        
    }
}

class Solution {
    List<Integer> list = new ArrayList<Integer>();
    public List<Integer> preorderTraversal(TreeNode root) {
        method(root);
        return list;
    }
    public void method(TreeNode root){
        if(root==null){
            return;
        }
        // 前序
        list.add(root.val);
        method(root.left);
        // 中序
        method(root.right);
        // 后序
    }
}