1309 - Sudoku

[sdipu]

Which method sould I use to solve this problem?

My attempt so far-
> Find all possible letters to place in a blank cell.
> Now take a cell which has minimum number of possible letters to place. [O(16*16*1000)]
> Backtrack by trying every possible value.

Method to find all possible letters-
> find all letters that can be placed in a cell without making any conflict with it's peer cells. [O(16)]
> hidden single check : check for a single letter to place in a cell, which doesn't contains in any other cells of same row, columns and box. [O(16*(15+15+15))]

I thought it would suffice. But it gives me TLE. I don't know any better solution. Please give me some help.