LeetCode 240： Search a 2D Matrix II

Write an efficient algorithm that searches for a value in an m x n matrix. This matrix has the following properties:

• Integers in each row are sorted in ascending from left to right.
• Integers in each column are sorted in ascending from top to bottom.

For example,

Consider the following matrix:

[  [1,   4,  7, 11, 15],  [2,   5,  8, 12, 19],  [3,   6,  9, 16, 22],  [10, 13, 14, 17, 24],  [18, 21, 23, 26, 30]]

Given target = 5, return true.

Given target = 20, return false.

此题与Search 2D Matrix基本相同，但现在每列也是有序的，可以采用从左下角或者右上角开始搜索的方法。

左下角：

class Solution {public:    bool searchMatrix(vector<vector<int>>& matrix, int target) {        int rows = matrix.size();if (rows<=0)return false;int cols = matrix[0].size();int i = rows-1;int j = 0;while(i>=0 && j<cols){if (matrix[i][j] == target)return true;else if (matrix[i][j]<target)j++;elsei--;}return false;    }};

右上角：

class Solution {public:    bool searchMatrix(vector<vector<int>>& matrix, int target) {        int rows = matrix.size();if (rows<=0)return false;int cols = matrix[0].size();int i=0, j=cols-1;while (i<rows && j>=0){int x = matrix[i][j];if (x == target){return true;}else if (x >target){j--;}else{i++;}}return false;    }};