LeetCode:Spiral Matrix

Spiral Matrix
Total Accepted: 53006 
Total Submissions: 240172 
Difficulty: Medium
Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order.
For example, Given the following matrix:

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

You should return  [1,2,3,6,9,8,7,4,5] .
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(M) Spiral Matrix II
code:

class Solution {
public:
    vector<int> spiralOrder(vector<vector<int>>& matrix) {
        vector<int> ret;
        if(matrix.empty() || 0==matrix.size()) return ret;
        int m = matrix.size();
        int n = matrix[0].size();
        
        int rowStart = 0, rowEnd = m-1;
        int colStart = 0, colEnd = n-1;
        while(rowStart <= rowEnd && colStart <= colEnd) {
            for(int i=colStart;i<=colEnd;i++) // left -> right
                ret.push_back(matrix[rowStart][i]);
            rowStart++;
            for(int i=rowStart;i<=rowEnd;i++) // up -> down
                ret.push_back(matrix[i][colEnd]);
            colEnd--;
            if(rowStart<=rowEnd) {
                for(int i=colEnd;i>=colStart;i--) // right -> left
                    ret.push_back(matrix[rowEnd][i]);
                rowEnd--;
            }
            if(colStart<=colEnd) {
                for(int i=rowEnd;i>=rowStart;i--) // down -> up
                    ret.push_back(matrix[i][colStart]);
                colStart++;
            }
        }
        return ret;
    }
};