二叉树篇

BST树

递归直接实现。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
          return dfs(nums,0,nums.length-1);
    }
    
    private TreeNode dfs(int [] nums,int left,int right){
        if(left>right){
            return null;
        }
        //以升序数组的中间元素作为根节点root
        int mid=(left+right)/2;
        TreeNode root = new TreeNode(nums[mid]);
        //构建root的左子树与右子树
        root.left=dfs(nums,left,mid-1);
        root.right=dfs(nums,mid+1,right);
        return root;
    }
}