题解 | JAVA #将升序数组转化为平衡二叉搜索树# [P0]

递归，分治
有序数组选中间点m作为root，recursiely build subtrees
m.left = build [l, m)
m.right = build [m+1, r)

public class Solution {
    public TreeNode sortedArrayToBST (int[] num) {
      if (num.length == 0) return null;
      return build(num, 0, num.length);
    }
  
    // build the BST for num[l:r)
    TreeNode build(int[] num, int l, int r) {
      if (l == r) return null;
      
      int m = l + ((r - l) >> 2);
      TreeNode n = new TreeNode(num[m]);
      n.left = build(num, l, m);
      n.right = build(num, m+1, r);
      
      return n;
    }
}