Search a 2D Matrix II
问题描述
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
.
解决思路
从最左下角的元素开始判断,最坏情况为遍历完矩阵的对角线上的元素,时间复杂度为O(n).
程序
public class Solution { public boolean searchMatrix(int[][] matrix, int target) { if (matrix == null || matrix.length == 0 || matrix[0].length == 0) { return false; } int rows = matrix.length, cols = matrix[0].length; // left bottom number int i = rows - 1, j = 0; while (i >= 0 && j <= cols - 1) { int num = matrix[i][j]; if (num == target) { return true; } if (num > target) { --i; continue; } if (num < target) { ++j; } } return false; } }