654. Maximum Binary Tree

构造root值最大的二叉树，先找最大值，再构造左右两边的子树。和前面的二分法一样，只是把求mid换成了找最大值的函数。

public TreeNode constructMaximumBinaryTree(int[] nums) {
    return dfs(nums, 0, nums.length - 1);
}

public TreeNode dfs(int[] nums, int l, int r) {
    if (l > r) return null;
    int max = max(nums, l, r);
    TreeNode root = new TreeNode(nums[max]);
    root.left = dfs(nums, l, max - 1);
    root.right = dfs(nums, max + 1, r);
    return root;
}

public int max(int[] nums, int l, int r) {
    int max = r;
    for (int i = l; i < r; i++) {
        if (nums[i] > nums[max]) max = i;
    }
    return max;
}