Search a 2D Matrix II

题目

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.

分析

显然，这个问题直接可以根据这个矩阵的特性来寻找target；
从第一排的最后一个开始，如果matrix[i][j] > target，就左移；如果发现matrix[i][j] < target 就下移；
这样时间复杂度只有O(m + n);

解答

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