把二维矩阵看成一棵二叉搜索树：

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class Solution {
 public:
  bool searchMatrix(vector<vector<int>>& matrix, int target) {
    int n = matrix.size();
    int r = 0, c = m - 1;
    while (r < n && c >= 0) {
      if (matrix[r][c] > target)
        c--;
      else if (matrix[r][c] < target)
        r++;
      else
        return true;
    }
    return false;
  }
};