代码随想录算法训练营Day14

翻转二叉树

class Solution {  
    public TreeNode invertTree(TreeNode root) {  
        if(root == null){  
            return null;  
        }  
        TreeNode temp = root.left;  
        root.left = root.right;  
        root.right = temp;  
        invertTree(root.left);  
        invertTree(root.right);  
        return root;  
    }  
}

对称二叉树

class Solution {  
    public boolean isSymmetric(TreeNode root) {  
        return compare(root.left,root.right);  
    }  
    public boolean compare(TreeNode left,TreeNode right){  
        if(left==null&&right!=null){  
            return false;  
        }else if(left!=null&&right==null){  
            return false;  
        }else if(left==null&&right==null){  
            return true;  
        }else if(left.val!=right.val){  
            return false;  
        }  
        boolean outside = compare(left.left,right.right);  
        boolean inside = compare(left.right,right.left);  
        return outside&&inside;  
    }  
}

二叉树最大深度

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

二叉树最小深度

这里有坑,注意排除根节点的左/右子节点为空的情况
最小深度是从根节点到叶子结点的距离的最小值.类似上面最大深度的判断方法

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