Leetcode P36周叮、38胎食、39
P36：
? ? ? ? Build a list including line number，column number and key value by two iterations. We can judge the validity of Sudoku by searching if duplicate elements in the list.?
P38：
? ? ? ? Use iteration to solve this problem. We need simulate the situations from 1 to n.
P39：
????????DFS can solve it easily!?
