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16// Java O(N^2)
class Solution {
public int[][] generateMatrix(int n) {
int[][] ret = new int[n][n];
int left = -1,right = n -1;
int top = 0,down = n - 1;
int count = 1;
while (left <= right) {
for (int i = ++left; i <= right; i++) ret[top][i] = count++;
for (int i = ++top; i <= down; i++) ret[i][right] = count++;
for (int i = --right; i >= left; i--) ret[down][i] = count++;
for (int i = --down; i >= top; i--) ret[i][left] = count++;
}
return ret;
}
}