EditeDistance

if x == y, then dp[i][j] == dp[i-1][j-1]
	    if x != y, and we insert y for word1, then dp[i][j] = dp[i][j-1] +
1
	    if x != y, and we delete x for word1, then dp[i][j] = dp[i-1][j] +
1
	    if x != y, and we replace x with y for word1, then dp[i][j] =
dp[i-1][j-1] + 1
	    When x!=y, dp[i][j] is the min of the three situations.

	    Initial condition:
	    dp[i][0] = i, dp[0][j] = j