左叶子之和

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

树最左下角的值

class Solution {
    public int findBottomLeftValue(TreeNode root) {
       if(root==null) return 0;
       int result=0;
       Queue<TreeNode> queue=new LinkedList<>();
       queue.add(root);
       while(!queue.isEmpty())
       {
           int t=queue.size();
           TreeNode temp;
           for(int i=0;i<t;i++)
           {
               temp=queue.poll();
               if(i==0) result=temp.val;
               if(temp.left!=null) queue.add(temp.left);
               if(temp.right!=null) queue.add(temp.right);
           }
       }
       return result;
    }
}

最大二叉树

class Solution {
    public TreeNode constructMaximumBinaryTree(int[] nums) {
        return construct(nums,0,nums.length-1);
    }
    public TreeNode construct(int[] nums,int ns,int ne)
    {
        if(ns>ne) return null;
        int index=ns;
        int curroot=Integer.MIN_VALUE;
        for(int i=ns;i<=ne;i++)
        {
            if(nums[i]>curroot)
            {
                curroot=Math.max(curroot,nums[i]);
                index=i;
            }
            
        }
        TreeNode root=new TreeNode(curroot);
        root.left=construct(nums,ns,index-1);
        root.right=construct(nums,index+1,ne);
        return root;
    }
}

合并二叉树

class Solution {
    public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
        if(root1==null) return root2;
        if(root2==null) return root1;
        TreeNode root=new TreeNode(root1.val+root2.val);
        root.left=mergeTrees(root1.left,root2.left);
        root.right=mergeTrees(root1.right,root2.right);
        return root;
    }
}

二叉树搜索树中的搜素

class Solution {
    public TreeNode searchBST(TreeNode root, int val) {
        if(root==null) return null;
        if(root.val<val)
        {
            return searchBST(root.right,val);
        }
        else if(root.val>val)
        {
            return searchBST(root.left,val);
        }
        else{
            return root;
        }
    }
}