https://leetcode-cn.com/problems/ju-zhen-zhong-de-lu-jing-lcof/

func exist(board [][]byte, word string) bool {
	n := len(word)
	r := len(board)
	c := len(board[0])
	if n > r*c {
		return false
	}
	g := func(p, q, a, b int) bool {
		return (p == a && (q-b == 1 || q-b == -1)) || ((p-a == 1 || p-a == -1) && q == b)
	}
	for i := 0; i < n; i++ {
		v := word[i]
		p, q, a, b := -1, -1, -1, -1
		f := false
		for j := 0; j < r; j++ {
			for k := 0; k < c; k++ {
				u := board[j][k]
				if v == u {
					p, q = j, k
					if a != -1 {
						if !g(p, q, a, b) {
							return false
						}
					}
					a, b = p, q
					f = true
					break
				}
			}
		}
		if !f {
			return true
		}
	}
	return true
}