class Solution {

public int minDistance(String w1, String w2) {

if(w1 == null || w2 == null) throw new IllegalArgumentException("xx");

int m = w1.length();

int n = w2.length();

int[][] t = new int[m+1][n+1];

for(int i = 0; i <= m; i++) {

for(int j = 0; j <= n; j++) {

if(i == 0) t[i][j] = j;

else if(j == 0) t[i][j] = i;

else {

if(w1.charAt(i-1) == w2.charAt(j-1)) t[i][j] = t[i-1][j-1];

else {

t[i][j] = Math.min(Math.min(t[i-1][j], t[i][j-1]), t[i-1][j-1]) +1;

}

}

}

}

return t[m][n];

}

}