Leetcode: Word Search

Remember to change the character back after loop through all the possibility

非常聪明的方法1：O（1）空间的话可以不用visited数组，直接改board。改了之后，再访问的时候因为board[i][j] != word.charAt(ind)所以会return false

相同的思路，非常聪明方法2：用一个bit mask，After board[y][x] ^= 256 the char became not a valid letter. After second board[y][x] ^= 256

it became a valid letter again.