251.展开二维向量

public class Vector2D{
    private int[][] matrix;
    private int row;
    private int col;
    private boolean curUse;
    public Vector2D(int[][] v) {
        matrix = v;
        row = 0;
        col = -1;
        curUse = true;
        hasNext();
    }
    public int next() {
        int ans = matrix[row][col];
        curUse = true;
        hasNext();
        return ans;
    }
    public boolean hasNext() {
        if (row == matrix.length) {
            return false;
        }
        // 重复多次调用弄hasNext的时候
        if (!curUse) {
            return true;
        }
        //(row, col)已经用过了
        if (col < matrix[row].length) {
            col++;
        } else {
            col = 0;
            do {
                row++;
            } while (row < matrix.length && matrix[row].length == 0);
        }
        //新的row， col
        if (row != matrix.length) {
            curUse = false;
            return true;
        } else {
            return false;
        }
    }
}