##状态转移
if a[i] == b[j]: DP[i][j] = min(DP[i][j], DP[i - 1][j - 1])#相等 DP[i][j] = min(DP[i - 1][j - 1] + 1, DP[i][j])#替换 DP[i][j] = min(DP[i - 1][j] + 1, DP[i][j])#插入a[i]or删除a[i] DP[i][j] = min(DP[i][j - 1] + 1, DP[i][j]) #插入b[j]or删除b[j]
##初始状态