[LeetCode] 106: Spiral Matrix II

[Problem]

Given an integer n, generate a square matrix filled with elements from 1 to n² in spiral order.

For example,
Given n = 3,

You should return the following matrix:

[ [ 1, 2, 3 ], [ 8, 9, 4 ], [ 7, 6, 5 ]]

[Solution]

class Solution {
public:
vector<vector<int> > generateMatrix(int n) {
// Note: The Solution object is instantiated only once and is reused by each test case.

// invalid
if(n <= 0){
vector<vector<int> > res;
return res;
}

// initial result
vector<vector<int> > res(n, vector<int>(n, 0));

// initial boder
int up = 0, bottom = n, left = -1, right = n, direct = 0;// 0: right, 1: down, 2: left, 3: up
int i = 0, j = 0, k = 1;
while(k <= n*n){
res[i][j] = k++;

// move right
if(direct == 0){
if(j+1 == right){
right--;
i++;
direct = 1;
}
else{
j++;
}
}
// move down
else if(direct == 1){
if(i+1 == bottom){
bottom--;
j--;
direct = 2;
}
else{
i++;
}
}
// move left
else if(direct == 2){
if(j-1 == left){
left++;
i--;
direct = 3;
}
else{
j--;
}
}
// move up
else if(direct == 3){
if(i-1 == up){
up++;
j++;
direct = 0;
}
else{
i--;
}
}
}
return res;
}
};

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