[ImgBot] Optimize images

*Total -- 367.65kb -> 289.86kb (21.16%)

/leetcode/backtracking/回溯算法.png -- 77.93kb -> 59.18kb (24.07%)
/leetcode/dynamic_programming/动态规划.png -- 178.64kb -> 141.33kb (20.88%)
/leetcode/greedy_algorithm/贪心算法.png -- 111.08kb -> 89.35kb (19.56%)

Signed-off-by: ImgBotApp <[email protected]>

Author: ImgBotApp